feat(messagestore): add Migrate4 with standalone FTS5 virtual table

Lays down a messages_fts virtual table with message_guid (UNINDEXED, hex
TEXT of the BLOB primary key), sender_text and content_text columns
using the unicode61 tokenizer with diacritic folding. Standalone FTS5
without content='messages' linking, because messages.Id is BLOB and
FTS5's content_rowid contract requires an INTEGER rowid alias.

LoadByGuids (Task 4.3) will resolve the hex GUIDs back to messages rows
via WHERE Id IN (...) joins. Schema step only -- the bulk-insert worker
that fills the index lives in Task 4.2.

Internal Connection property exposure plus a HasMessagesFtsTable helper
let the Build-Suite verify Migrate4 without raw PRAGMA glue in each test.

v1.4.8 H2 Sub-Task 4.1.

HellionChat/MessageStore.cs

@@ -126,7 +126,12 @@ internal class MessageStore : IDisposable
     private const int MessageQueryLimit = 10_000;
 
     private string DbPath { get; }
-    private SqliteConnection Connection { get; set; }
+
+    // Internal so the Build-Suite tests can verify Migrate4's CREATE VIRTUAL
+    // TABLE result via a one-off PRAGMA without exposing a dedicated helper
+    // for each schema invariant. Setter stays private; the ctor is the only
+    // place that assigns.
+    internal SqliteConnection Connection { get; private set; }
 
     internal static readonly MessagePackSerializerOptions MsgPackOptions =
         MessagePackSerializerOptions.Standard.WithResolver(
@@ -227,13 +232,19 @@ internal class MessageStore : IDisposable
                 migrationsToDo.Add(Migrate1);
                 migrationsToDo.Add(Migrate2);
                 migrationsToDo.Add(Migrate3);
+                migrationsToDo.Add(Migrate4);
                 break;
             case 1:
                 migrationsToDo.Add(Migrate2);
                 migrationsToDo.Add(Migrate3);
+                migrationsToDo.Add(Migrate4);
                 break;
             case 2:
                 migrationsToDo.Add(Migrate3);
+                migrationsToDo.Add(Migrate4);
+                break;
+            case 3:
+                migrationsToDo.Add(Migrate4);
                 break;
         }
 
@@ -344,6 +355,30 @@ internal class MessageStore : IDisposable
         SetMigrationVersion(3);
     }
 
+    private void Migrate4()
+    {
+        _logger.Information("Running migration 4: Add FTS5 virtual table for full-text search");
+
+        // Standalone FTS5 table (no content='messages' linking, no content_rowid).
+        // messages.Id is BLOB-PK (Guid), which is incompatible with FTS5's
+        // content_rowid requirement of an INTEGER rowid alias. We store the
+        // GUID as a hex TEXT column (UNINDEXED so the tokenizer skips it) and
+        // FTS5 manages its own internal INTEGER rowid. LoadByGuids joins back
+        // via WHERE Id IN (... unhex(message_guid)) when the search returns.
+        using var cmd = Connection.CreateCommand();
+        cmd.CommandText = """
+            CREATE VIRTUAL TABLE IF NOT EXISTS messages_fts USING fts5(
+                message_guid UNINDEXED,
+                sender_text,
+                content_text,
+                tokenize='unicode61 remove_diacritics 2'
+            );
+            """;
+        cmd.ExecuteNonQuery();
+
+        SetMigrationVersion(4);
+    }
+
     private void SetMigrationVersion(int version)
     {
         _logger.Information($"Setting version {version}");
@@ -493,6 +528,16 @@ internal class MessageStore : IDisposable
         return Convert.ToInt32(cmd.ExecuteScalar());
     }
 
+    // Schema probe for the v1.4.8 FTS5 virtual table. Used by the Build-Suite
+    // tests to verify Migrate4's CREATE VIRTUAL TABLE actually landed without
+    // duplicating PRAGMA glue in each test body.
+    internal bool HasMessagesFtsTable()
+    {
+        using var cmd = Connection.CreateCommand();
+        cmd.CommandText = "SELECT count(*) FROM sqlite_master WHERE name='messages_fts';";
+        return (long)(cmd.ExecuteScalar() ?? 0L) > 0;
+    }
+
     internal void UpsertMessage(Message message)
     {
         // Privacy filter -- drop disallowed ChatTypes before they reach storage.