Sync credentials as a vault item type, and bind one to a host

Closes the largest remaining M1 gap in the data layer: a username and password
can live in the vault, sync between machines, and be named by a host as how it
authenticates. What is not here is the interface for creating one — see the end
of this message.

The third item type, and the first one that cost almost nothing to add. Server:
a VaultCredential row, an EF configuration, a migration, and a CredentialKind.
Client: a secret, a codec, a merge, a cipher, a kind, a repository facade and a
session property. No new reconciliation logic, no change to the sync engine, no
client cache migration. That was the whole point of the item-kind seam, and this
is the evidence it holds.

The narrowest type of the three on plaintext, and not for symmetry. A host has a
deliberate concession — the relay needs an address it can resolve. A key has a
fingerprint, public by nature, which this client still declines to send. A
password has no part that is safe to expose: not its length, not a hash, not a
hint. So CredentialKind refuses every plaintext field there is, hydrates none,
and the table has no column to put one in.

HostSecret.CredentialId is the password counterpart of SshKeyId, and the two are
mutually exclusive. SSH itself would happily try a key and fall back to a
password, but a host naming both leaves "how does this authenticate?" without a
single answer — the interface, the connect path and the user would each be free
to guess differently. TryValidate refuses it. One consequence was not
anticipated: "a full host" stops being a coherent idea, which is what broke
AFullHost_RoundTrips and is now written into that test.

The schema version became a ladder rather than a maximum: credential-bound is 3,
key-bound is 2, neither is still 1. Adding credentials therefore does not drag
every key-bound host in every vault onto a version that clients understanding
keys perfectly well would refuse to edit. A test pins exactly that, because it is
the property the whole content-dependent-version rule exists to provide, and the
obvious implementation would quietly lose it.

Two tests had become false and said so:

- Push_AnUnsupportedEntityType_IsInvalidNotAFailedBatch used Credential as its
  example of a type this server does not implement. It now asks the server's own
  registry what is still missing, so it cannot go stale again, and skips with a
  reason if that set ever empties.
- ThePullFilterNamesEveryTypeThisBuildSynchronises pinned the exact list, which
  is what it is for.

Also fixes ten nullable warnings — eight in SyncEndpointTests, two in a test file
added earlier today. Neither set was introduced here; both were invisible until
an unrelated change forced their project to recompile, which means the
zero-warning claims made earlier in this work only ever covered what happened to
be rebuilt.

777 tests green. Zero warnings, dotnet format clean.

Not done, and deliberately: the credential interface. The vault column is 340
pixels wide and already holds two lists and two editors, kept from clipping its
own buttons at the window's minimum height only by the one-editor-at-a-time rule
added earlier today. A third list and a third editor would recreate that defect
rather than avoid it, so the column needs a shape decision first. Credentials
sync; they cannot yet be created in the interface.

tests/DodoSSH.Client.Domain.Tests/HostSecretCodecTests.cs

@@ -14,11 +14,17 @@ namespace DodoSSH.Client.Domain.Tests;
 /// </remarks>
 public sealed class HostSecretCodecTests
 {
+    /// <remarks>
+    /// "Full" cannot mean every field any more: the two bindings are mutually exclusive, so a host may carry
+    /// a key or a credential and never both. This one carries the credential because that is the newer of
+    /// the two and therefore the highest schema version a valid host can reach; the key-bound case has its
+    /// own version test below.
+    /// </remarks>
     [Fact]
     public void AFullHost_RoundTrips()
     {
-        // Every field, which is what makes the version assertion below meaningful: a host carrying the
-        // newest field is the only kind written at the newest version.
+        var credentialId = Guid.Parse("0192f0c8-5555-7c3d-8e4f-5a6b7c8d9e05");
+
         var host = Host(
             label: "prod-db",
             hostname: "db.internal",
@@ -28,12 +34,13 @@ public sealed class HostSecretCodecTests
             jumps: [Bastion, Relay],
             options: [("ServerAliveInterval", "30"), ("Compression", "yes")],
             relayEnabled: true,
-            sshKeyId: DeployKey);
+            credentialId: credentialId);
 
         HostSecretCodec.TryDecode(HostSecretCodec.Encode(host), out var document).ShouldBeTrue();
 
         document.ShouldNotBeNull();
         document.Host.ShouldBe(host);
+        document.Host.CredentialId.ShouldBe(credentialId);
         document.SchemaVersion.ShouldBe(HostSecretCodec.CurrentSchemaVersion);
         document.IsReadOnly.ShouldBeFalse();
     }
@@ -65,6 +72,37 @@ public sealed class HostSecretCodecTests
         document.Host.SshKeyId.ShouldBe(DeployKey);
     }
 
+    [Fact]
+    public void AHostThatBindsACredential_IsWrittenAtTheVersionThatIntroducedIt()
+    {
+        // Each binding earns its own version, so a host using only the older one is not dragged forward onto
+        // a version older clients refuse to edit.
+        var credentialId = Guid.CreateVersion7();
+
+        HostSecretCodec
+            .TryDecode(HostSecretCodec.Encode(Host(credentialId: credentialId)), out var document)
+            .ShouldBeTrue();
+
+        document.ShouldNotBeNull();
+        document.SchemaVersion.ShouldBe(HostSecretCodec.CredentialIdSchemaVersion);
+        document.Host.CredentialId.ShouldBe(credentialId);
+    }
+
+    [Fact]
+    public void AKeyBoundHost_IsNotDraggedOntoTheCredentialVersion()
+    {
+        // The point of the ladder. Adding credentials must not make every key-bound host in every vault
+        // read-only on a client that understands keys perfectly well.
+        HostSecretCodec
+            .TryDecode(HostSecretCodec.Encode(Host(sshKeyId: DeployKey)), out var document)
+            .ShouldBeTrue();
+
+        var version = document.ShouldNotBeNull().SchemaVersion;
+
+        version.ShouldBe(HostSecretCodec.SshKeyIdSchemaVersion);
+        version.ShouldBeLessThan(HostSecretCodec.CredentialIdSchemaVersion);
+    }
+
     [Fact]
     public void AddingTheKeyField_DidNotChangeTheBytesOfAHostWithoutOne()
     {