Add SQLite FTS5 scrollback index (no UI yet)

[buggerman]

Summary

• First half of scrollback search: a SQLite + FTS5 index that mirrors the writes ScrollbackStore already performs to per-channel JSONL. Adds the indexer, the search/clear API, and idempotent cold-start backfill that runs through the existing rehydrate path. No UI in this PR — that lands in a follow-up so this can be reviewed and reverted independently.
• Pure-Swift policy is honored under the carveout merged in Allow imports of system C modules (e.g. SQLite3) in AGENTS.md #28: the unsafe sqlite3_* C surface lives entirely inside an actor and the public API takes/returns Swift value types only (Message, UUID, Date, String).
• Phrase, prefix, boolean, and column-scoped queries come for free via FTS5 ("foo bar", bar*, term1 AND term2 NOT term3, sender:alice).

Changes

• Sources/BryggaCore/Persistence/ScrollbackIndex.swift — new file. Public actor ScrollbackIndex with index(_:serverID:target:), search(_:scope:limit:), and clear(serverID:target:). Public SearchScope enum and SearchHit struct. Schema: one messages FTS5 virtual table with sender/content indexed and msg_id/server_id/target/timestamp/kind UNINDEXED, tokenizer unicode61 remove_diacritics 2. Database opens lazily on first call; PRAGMA journal_mode=WAL + synchronous=NORMAL. Idempotent on msg_id so cold-start replay never double-counts.
• Sources/BryggaCore/IRC/IRCSession.swift — three new index sites alongside the existing ScrollbackStore.append: record(_:in:), recordServer(_:), and the chathistory finalize loop. loadScrollbackIfNeeded(for:) now also indexes loaded messages so channels first seen post-launch (JOIN, openQuery) backfill into the index.
• Sources/BryggaCore/Models/AppState.swift — restoreFromStore's rehydrate task indexes every loaded message after insertion (cold-start backfill on the canonical channel name). closePrivateMessage and removeServer now clear the index for the closed scope. Backfill is "lazy through the rehydrate loop" rather than a filesystem walker, so we always use real channel names instead of the sanitized JSONL filenames.
• Tests/ScrollbackIndexTests.swift — new file. 10 unit tests covering single-message search, idempotency, server/channel scope filtering, clear semantics, FTS5 phrase + prefix + column-scoped queries, timestamp/kind round-trip, and empty-query handling. Each test uses :memory: for full isolation.

Test plan

• swift build — passes
• swift test — 131 tests pass (+10 new)
• swiftformat --lint . — clean
• Manual:
  • Connect to a running ircd, exchange a few messages, then in an lldb session call await ScrollbackIndex.shared.search("...") to confirm the index is being populated.
  • Verify ~/Library/Application Support/Brygga/scrollback.sqlite is created and contains rows: sqlite3 scrollback.sqlite 'SELECT count(*) FROM messages'.
  • Restart the app — confirm the rehydrate path back-fills the index for previously-seen channels (idempotent: count should not double).
  • Close a PM (Cmd+W) and confirm its rows are gone from the index. Same for "Remove Server".

Risk / rollback

• New file at ~/Library/Application Support/Brygga/scrollback.sqlite. Disk footprint scales with scrollback volume; FTS5 typically uses ~1.5–2× the indexed text size. Safe to delete the file at any time — the next launch's rehydrate will repopulate.
• All write sites are fire-and-forget after the existing ScrollbackStore.append so a failing index write can't break the existing JSONL pipeline.
• The unsafe pointer wrapping is contained inside the actor — OpaquePointer and sqlite3_* calls are not visible to the rest of the codebase.
• No deinit-time cleanup (would require isolated deinit from macOS 15.4); production singleton never deinits, :memory: test instances cleaned up at process exit. Documented in-source.
• Revert: this single commit. The scrollback.sqlite file becomes orphaned on disk after revert; users can delete it manually with no impact on chat history (which still lives in JSONL).