Give the two logs and the buckets a resource type, so a conflict can be written

AadResourceTypes.For maps a syncable type onto the AAD resource type its cache
records bind to, and it had no arm for ConnectionLogEntry, ActivityLogEntry or
ObjectStore. All three are on both enums, in the reconciler registry and in the
cipher pinning; only this switch was missed, and it throws rather than falling
back — so a merge conflict on a connection log, an activity log or a bucket
raised ArgumentOutOfRangeException on the path that records what the merge
discarded. The conflict log is the whole reason the merge is allowed to pick a
winner, so the one item kind whose conflicts could not be recorded was a bucket:
an editable item two machines can genuinely disagree about.

Worth writing down why it lasted two phases. Of the three callers, ItemStore and
OutboxStore reach the mapping only when an item carries plaintext fields, and
none of these three kinds does — so they never touched the gap. ConflictStore
calls it unconditionally, but a test only reaches that by causing a real merge
conflict, and every existing one raised its conflict against a Host. Three arms
missing, and no path in the suite crossed any of them.

So the tests are the point of this commit as much as the arms are. The guard is
AadResourceTypeTests.EverySyncableType_HasAnArmInTheStorageMapping: it walks the
whole wire enum, and for each type asserts both that there is an arm and that the
arm returns the same-named resource type, which is the mistake the file's cipher
half already guards against on the server side. Written over the full enum rather
than over ItemKinds.SyncedTypes, because that is the stronger claim and the one
the switch really makes — the two reserved association types have arms too.
Beside it, CacheStoreTests.AConflict_CanBeRecordedForEveryKindOfItem records a
conflict per kind and reads the detail back, since an arm returning the wrong
resource type seals under one AAD and opens under another, which surfaces as an
empty detail rather than as a throw.

Both were confirmed to fail with the arms removed: the theory fails on exactly
ConnectionLogEntry, ActivityLogEntry and ObjectStore and passes on the other
three, and the guard names those three and no others.

The note in docs/adding-hosts-on-the-phone.md that recorded this as out of scope
is marked fixed, with what let it survive, since that is the part worth knowing
next time an item kind is added.

1529 tests pass, seven of them new.

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

@@ -1,5 +1,6 @@
 using System.Security.Cryptography;
 using DodoSSH.Client.Domain;
+using DodoSSH.Client.Storage;
 using DodoSSH.Client.Sync;
 using DodoSSH.Contracts;
 using DodoSSH.Crypto;
@@ -151,6 +152,63 @@ public sealed class AadResourceTypeTests
             .ShouldBe(ItemKinds.SyncedTypes, ignoreOrder: true);
     }
 
+    /// <summary>
+    /// The same pairing, made a second time in the storage layer, and every type must be in it.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <c>AadResourceTypes.For</c> is the cache's copy of the table above: the ciphers seal an item for the
+    /// <em>server</em>, and this seals the two things the local cache holds in the clear — a relay host's
+    /// address, and the values a merge overrode. A type missing from it throws rather than mis-seals, which
+    /// sounds like the safe failure and is not: <c>ConflictStore.RecordAsync</c> calls it unconditionally, so
+    /// the exception lands on the path that records what a merge discarded.
+    /// </para>
+    /// <para>
+    /// This is written after finding three types missing from it — <c>ConnectionLogEntry</c>,
+    /// <c>ActivityLogEntry</c> and <c>ObjectStore</c> went two shipping phases without an arm, because the
+    /// only unconditional caller is one a test suite reaches solely by causing a real merge conflict. Asserted
+    /// over the whole wire enum rather than over <c>ItemKinds.SyncedTypes</c>, which is the stronger claim and
+    /// the one the switch actually makes: the two reserved association types have arms too.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public void EverySyncableType_HasAnArmInTheStorageMapping()
+    {
+        var missing = new List<SyncEntityType>();
+
+        foreach (var wire in Enum.GetValues<SyncEntityType>())
+        {
+            if (wire == SyncEntityType.Unspecified)
+            {
+                continue;
+            }
+
+            CryptoSpec.AadResourceType resource;
+
+            try
+            {
+                resource = AadResourceTypes.For(wire);
+            }
+            catch (ArgumentOutOfRangeException)
+            {
+                missing.Add(wire);
+                continue;
+            }
+
+            // Same name, as the cipher table demands — a wrong-but-present arm is the failure this half
+            // would otherwise wave through.
+            Enum.GetName(resource).ShouldBe(
+                Enum.GetName(wire),
+                $"AadResourceTypes.For({wire}) returns {resource}, which binds this type's cache records "
+                + "to another type's resource.");
+        }
+
+        missing.ShouldBeEmpty(
+            "every syncable type needs an arm in AadResourceTypes.For, or a conflict recorded against one "
+            + "of these throws instead of being written — and the conflict log is what justifies the merge "
+            + "picking a winner.");
+    }
+
     private static EncryptedPayload SealSample(
         SyncEntityType wire,
         byte[] vaultKey,