Sync and authenticate with SSH keys on the client

Completes the client half of SSH keys: they sync alongside hosts, appear in
their own list, and can be selected to authenticate a connection instead of
typing a password.

The reconciler and the repository were Host-typed throughout, so the choice was
to generalise them or to keep a second copy per item type. Generalised, because
ItemReconciler's whole premise is that the pull and the push paths must answer
the same collision the same way — two copies would drift the first time one of
them was fixed. What is genuinely per-type now arrives through
IItemKind<TSecret>: the cipher, the merge, the plaintext columns, and the noun
to use when telling a person what happened to their item. Generic where the
server's IItemKind is not, and for the reason that reverses there — the client
needs the concrete type, because it merges field by field.

The pull filter is derived from the same registry that builds the reconcilers.
That is the specific failure being designed out: an item type that encrypts,
merges and lists perfectly and is never once requested from the server, so it
works on the machine that made it and exists nowhere else.

No client cache migration. The item table's primary key and the outbox's unique
index already carry the entity type, and AadResourceTypes already mapped SshKey
— so a host and a key may share an id and never see each other's rows, which
SshKeySyncTests now arranges deliberately.

A key hands the server nothing in plaintext. There is a public_key_fingerprint
column and it would be accepted; leaving it null is deliberate. A fingerprint is
not secret but it is a stable identifier for a key pair, so filling it would let
an operator tell which of their users hold the same key and correlate one across
vaults, for a column nothing reads. The design allows itself one plaintext
concession — the relay address, which the relay cannot work without — and this
is not that.

A key is chosen per connection rather than bound to a host, which works the way
ssh -i does. Binding one needs a field on HostSecret and therefore a payload
schema bump, which makes every host written afterwards read-only on an older
build; worth doing deliberately rather than as a side effect of adding keys.

Three things this found, all of them by being falsified rather than by review:

- Making the reconciler generic silently turned a record comparison into
  reference equality, because == on a type parameter is not value equality. The
  effect would have been a conflict recorded on every pass for an unacknowledged
  create that had in fact landed. Sabotaging the fix left all 73 tests passing —
  nothing covered that branch — so ConflictMatrixTests now has
  AnUnacknowledgedCreateThatDidLand_IsDroppedQuietly, which fails without it.

- A test asserting that a blank passphrase reaches SSH.NET as null was vacuous:
  it exercised the editor, not the credential path, and passed with the guard
  deleted. Resolved by making SshKeySecret.Passphrase normalise an empty string
  to null, so there is one spelling of one state — which also keeps two clients
  from producing different payload bytes for an identical key. That exposed a
  wider gap: SshKeySecret, its codec and its merge had no direct unit tests at
  all. They have 25 now.

- The reason first given for that normalisation was false. It claimed SSH.NET
  rejects a passphrase supplied for an unprotected key; measured against a real
  sshd it ignores it and authenticates anyway. Corrected everywhere it was
  stated and recorded in docs/platform-flags.md. The same test file also closes
  a real hole: SshPrivateKeyCredential had never been exercised against a
  server, because the existing key test builds SSH.NET's auth method directly
  and bypasses the path a vault-held key actually takes.

Only one editor may be open at a time. Both sit in the same 340-pixel column as
Auto rows and their heights together exceed it at the window's minimum size, so
two open editors put the lower one's Save and Cancel past the bottom edge — the
same failure this window already shipped once with the setup screens. Expressed
as a state rule because that is the only form of it this repository can check:
nothing here loads a .axaml. The refusal keeps what was typed, since in the key
editor that is a pasted private key the user may have nowhere else.

The end-to-end slice now carries a key as well as a host, so both item types go
through the real API, the real PostgreSQL and the real crypto in one pass — the
three hand-kept mappings between enums that do not line up are the reason that
is worth doing rather than trusting the unit suites.

735 tests green, including the container-backed SSH and end-to-end suites. Zero
warnings, dotnet format clean.

tests/DodoSSH.Client.Sync.Tests/FakeVaultServer.cs

@@ -11,19 +11,28 @@ namespace DodoSSH.Client.Sync.Tests;
 /// <para>
 /// A faithful reimplementation of <c>DodoSSH.Api.Features.Sync.SyncService</c>'s decision table: the
 /// version check, the tombstone-beats-late-upsert rule, idempotent deletes, operation receipts, the
-/// change log, and cursors that are opaque to the client. It is not a stub that returns canned answers —
-/// if it were, none of the conflict tests would mean anything, because the interesting behaviour is
-/// exactly the server's refusal to apply a stale write.
+/// change log, cursors that are opaque to the client, the pull filter, and the per-type rules about which
+/// plaintext columns an item may carry. It is not a stub that returns canned answers — if it were, none of
+/// the conflict tests would mean anything, because the interesting behaviour is exactly the server's
+/// refusal to apply a stale write.
 /// </para>
 /// <para>
 /// The duplication against the real service is deliberate and is the point of the exercise: two
 /// independent expressions of the same rules, and <c>SyncEndpointTests</c> checks the other one against
 /// real Postgres. A shared implementation would let a misreading of the protocol pass on both sides.
 /// </para>
+/// <para>
+/// Rows are keyed on the entity type as well as the id, as the server's separate tables are and as the
+/// client's cache is. Keying on the id alone would work for every test that uses one item type and would
+/// silently make a host and a key with the same id the same row.
+/// </para>
 /// </remarks>
 internal sealed class FakeVaultServer : ISyncApi
 {
-    private readonly Dictionary<Guid, Row> rows = [];
+    /// <summary>The item types this fake knows, mirroring the server's own registry.</summary>
+    private static readonly SyncEntityType[] Supported = [SyncEntityType.Host, SyncEntityType.SshKey];
+
+    private readonly Dictionary<(SyncEntityType Type, Guid EntityId), Row> rows = [];
     private readonly List<LogEntry> log = [];
     private readonly Dictionary<Guid, Receipt> receipts = [];
 
@@ -49,6 +58,9 @@ internal sealed class FakeVaultServer : ISyncApi
     /// <summary>Pushes received, so a test can prove a retry did or did not happen.</summary>
     internal int PushCount { get; private set; }
 
+    /// <summary>The entity-type filter of the last pull, so a test can assert what was asked for.</summary>
+    internal IReadOnlyList<SyncEntityType>? LastPullTypes { get; private set; }
+
     /// <summary>
     /// Runs just before a push is applied, so a test can land another client's write in the window
     /// between one client's pull and its push. That window is the whole subject of the cursor-gap test.
@@ -66,7 +78,16 @@ internal sealed class FakeVaultServer : ISyncApi
         var after = DecodeCursor(request.Cursor);
         var limit = Math.Clamp(request.Limit ?? MaxPullLimit, 1, MaxPullLimit);
 
-        var page = log.Where(entry => entry.Sequence > after).Take(limit + 1).ToList();
+        LastPullTypes = request.EntityTypes;
+
+        // Empty or absent means every type, as the contract says.
+        var wanted = request.EntityTypes is { Count: > 0 } types ? types : null;
+
+        var page = log
+            .Where(entry => entry.Sequence > after)
+            .Where(entry => wanted is null || wanted.Contains(entry.EntityType))
+            .Take(limit + 1)
+            .ToList();
 
         var hasMore = page.Count > limit;
         if (hasMore)
@@ -109,18 +130,22 @@ internal sealed class FakeVaultServer : ISyncApi
     }
 
     /// <summary>Applies a change as if another client had made it.</summary>
-    internal int ExternalUpsert(Guid entityId, EncryptedPayload payload, SyncPlaintextFields? fields)
+    internal int ExternalUpsert(
+        Guid entityId,
+        EncryptedPayload payload,
+        SyncPlaintextFields? fields,
+        SyncEntityType entityType = SyncEntityType.Host)
     {
         var result = Apply(new SyncPushOperation(
             Guid.CreateVersion7(),
-            SyncEntityType.Host,
+            entityType,
             entityId,
             SyncOperation.Upsert,
-            rows.TryGetValue(entityId, out var existing) && !existing.IsDeleted
+            rows.TryGetValue((entityType, entityId), out var existing) && !existing.IsDeleted
                 ? existing.Version
                 : null,
             payload,
-            fields ?? new SyncPlaintextFields()));
+            fields));
 
         if (result.Status != SyncOperationStatus.Applied)
         {
@@ -132,13 +157,13 @@ internal sealed class FakeVaultServer : ISyncApi
     }
 
     /// <summary>Deletes as if another client had done it.</summary>
-    internal void ExternalDelete(Guid entityId)
+    internal void ExternalDelete(Guid entityId, SyncEntityType entityType = SyncEntityType.Host)
     {
-        var existing = rows[entityId];
+        var existing = rows[(entityType, entityId)];
 
         var result = Apply(new SyncPushOperation(
             Guid.CreateVersion7(),
-            SyncEntityType.Host,
+            entityType,
             entityId,
             SyncOperation.Delete,
             existing.Version,
@@ -151,7 +176,8 @@ internal sealed class FakeVaultServer : ISyncApi
         }
     }
 
-    internal Row? Find(Guid entityId) => rows.TryGetValue(entityId, out var row) ? row : null;
+    internal Row? Find(Guid entityId, SyncEntityType entityType = SyncEntityType.Host) =>
+        rows.TryGetValue((entityType, entityId), out var row) ? row : null;
 
     private long Head => log.Count == 0 ? 0 : log[^1].Sequence;
 
@@ -159,7 +185,7 @@ internal sealed class FakeVaultServer : ISyncApi
 
     private SyncPushResult Apply(SyncPushOperation operation)
     {
-        if (operation.EntityType != SyncEntityType.Host)
+        if (!Supported.Contains(operation.EntityType))
         {
             return Invalid(operation, $"Entity type {operation.EntityType} is not yet supported.");
         }
@@ -181,7 +207,7 @@ internal sealed class FakeVaultServer : ISyncApi
                 operation.OperationId, SyncOperationStatus.Forbidden, null, null, null, null);
         }
 
-        rows.TryGetValue(operation.EntityId, out var existing);
+        rows.TryGetValue((operation.EntityType, operation.EntityId), out var existing);
 
         return operation.Operation == SyncOperation.Delete
             ? ApplyDelete(operation, existing)
@@ -202,14 +228,9 @@ internal sealed class FakeVaultServer : ISyncApi
 
         var fields = operation.PlaintextFields ?? new SyncPlaintextFields();
 
-        if (!fields.RelayEnabled && (fields.Hostname is not null || fields.Port is not null))
+        if (!ValidateFields(operation.EntityType, fields, out var fieldError))
         {
-            return Invalid(operation, "An address may only be supplied when relay is enabled.");
-        }
-
-        if (fields.RelayEnabled && (string.IsNullOrWhiteSpace(fields.Hostname) || fields.Port is null))
-        {
-            return Invalid(operation, "Relay-enabled hosts require both a hostname and a port.");
+            return Invalid(operation, fieldError);
         }
 
         if (existing is null || existing.IsDeleted)
@@ -233,6 +254,45 @@ internal sealed class FakeVaultServer : ISyncApi
         return Commit(operation, updated, SyncOperation.Upsert);
     }
 
+    /// <summary>The per-type rules about which plaintext columns an item may carry.</summary>
+    /// <remarks>
+    /// A key's are stricter than a host's rather than merely different, and that asymmetry is the point:
+    /// the relay concession belongs to hosts alone, so a key arriving with an address is a client bug and
+    /// is refused with a reason instead of being quietly dropped.
+    /// </remarks>
+    private static bool ValidateFields(
+        SyncEntityType entityType,
+        SyncPlaintextFields fields,
+        out string error)
+    {
+        error = string.Empty;
+
+        if (entityType == SyncEntityType.SshKey)
+        {
+            if (fields.RelayEnabled || fields.Hostname is not null || fields.Port is not null)
+            {
+                error = "An SSH key has no relay target; relay fields may only be set on a host.";
+                return false;
+            }
+
+            return true;
+        }
+
+        if (!fields.RelayEnabled && (fields.Hostname is not null || fields.Port is not null))
+        {
+            error = "An address may only be supplied when relay is enabled.";
+            return false;
+        }
+
+        if (fields.RelayEnabled && (string.IsNullOrWhiteSpace(fields.Hostname) || fields.Port is null))
+        {
+            error = "Relay-enabled hosts require both a hostname and a port.";
+            return false;
+        }
+
+        return true;
+    }
+
     private SyncPushResult Create(SyncPushOperation operation, Row? existing, SyncPlaintextFields fields)
     {
         // A tombstone beats a late upsert. The client is told so it can resurrect the item deliberately
@@ -248,7 +308,9 @@ internal sealed class FakeVaultServer : ISyncApi
             return Conflict(operation, existing: null);
         }
 
-        var created = new Row(operation.EntityId, 1, 0, operation.Payload!, fields, false);
+        var created = new Row(
+            operation.EntityType, operation.EntityId, 1, 0, operation.Payload!, fields, false);
+
         return Commit(operation, created, SyncOperation.Upsert);
     }
 
@@ -293,8 +355,8 @@ internal sealed class FakeVaultServer : ISyncApi
     {
         var sequence = Head + 1;
 
-        log.Add(new LogEntry(sequence, row.EntityId, change, row.Version, Now));
-        rows[row.EntityId] = row with { ChangeSequence = sequence };
+        log.Add(new LogEntry(sequence, row.EntityType, row.EntityId, change, row.Version, Now));
+        rows[(row.EntityType, row.EntityId)] = row with { ChangeSequence = sequence };
         receipts[operation.OperationId] = new Receipt(row.Version, sequence);
 
         return new SyncPushResult(
@@ -315,13 +377,13 @@ internal sealed class FakeVaultServer : ISyncApi
 
     private SyncChange Hydrate(LogEntry entry)
     {
-        var row = rows[entry.EntityId];
+        var row = rows[(entry.EntityType, entry.EntityId)];
         return ToChange(row, entry.Sequence, entry.Revision, entry.OccurredAt);
     }
 
     private SyncChange ToChange(Row row, long? sequence = null, int? version = null, DateTimeOffset? at = null) =>
         new(
-            SyncEntityType.Host,
+            row.EntityType,
             row.EntityId,
             row.IsDeleted ? SyncOperation.Delete : SyncOperation.Upsert,
             version ?? row.Version,
@@ -359,6 +421,7 @@ internal sealed class FakeVaultServer : ISyncApi
     }
 
     internal sealed record Row(
+        SyncEntityType EntityType,
         Guid EntityId,
         int Version,
         long ChangeSequence,
@@ -368,6 +431,7 @@ internal sealed class FakeVaultServer : ISyncApi
 
     private sealed record LogEntry(
         long Sequence,
+        SyncEntityType EntityType,
         Guid EntityId,
         SyncOperation Operation,
         int Revision,