music: recursive watcher → localized reindex on any change

Extract walk()'s per-folder logic into a reusable buildFolder(), so a single
album/artist can be (re)indexed on its own. Add:

- reindexFolder(rel): rebuild just one folder's live cache entry + patch the
  manifest (prune if the folder vanished). Its mtime-based signature means any
  change — add / re-tag / delete — is picked up, and irrelevant touches no-op.
- withIndexLock: serialize ALL index mutations (full / incremental / localized)
  so a localized reindex can never race the full reindex's atomic swap.
- watcher.ts: fs.watch(~/Music, { recursive }) → log every change → debounce 3s →
  reindexFolder the affected folder(s). Verified: Bun's recursive watch fires
  through the ~/Music symlink and on new-dir creation (so a new album's contents
  are read by reindexFolder); inotify max_user_watches (~483k) >> folder count.

Started at boot, stopped on shutdown. The nightly full reindex backstops any
change that lands after a new folder's debounce.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/music/index.ts

@@ -4,6 +4,7 @@ import type { SidecarCommand, SidecarEvent } from '../protocol';
 import { createSidecarConnector } from '../connect';
 import { streamAudioFile } from './stream-audio';
 import { startNightlyReindex, stopNightlyReindex } from './nightly-reindex';
+import { startMusicWatcher, stopMusicWatcher } from './watcher';
 import {
   reindexNow,
   ensureCacheSetup,
@@ -78,6 +79,9 @@ await ensureCacheSetup();
 // Nightly full reindex at 3am (staged + atomic swap).
 startNightlyReindex();
 
+// Recursive watcher on ~/Music → localized reindex on any change.
+startMusicWatcher();
+
 const server = Bun.serve({
   port,
   hostname: '127.0.0.1',
@@ -272,6 +276,7 @@ const connection = createSidecarConnector({
 function shutdown(signal: string) {
   console.log(`[music] ${signal} received, shutting down...`);
   stopNightlyReindex();
+  stopMusicWatcher();
   try {
     server.stop(true);
   } catch {