music: report what the nightly full actually found

The nightly full rebuilds every album by definition, so "it completed" says
nothing about whether it is still needed. Diff the from-scratch build against
the index that was already live, just before the slot swaps in, and log the
delta: albums added, removed, and entries whose contents changed.

A run reporting NONE did no useful work. A string of those is the evidence for
retiring the nightly; a delta that keeps coming back names the albums to go and
look at instead of guessing.

Entries are compared field by field rather than by JSON.stringify: the optional
fields are spread conditionally, so two entries built by the same code can
serialise with different key order, and stringify would report every album as
changed every night. A cache-format upgrade is labelled, since that rebuilds
everything legitimately and would otherwise read as total rot.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/servers/sidecar/music/indexer.ts

@@ -645,6 +645,57 @@ export async function reindexNow(): Promise<IndexStatus> {
   });
 }
 
+type ManifestDelta = { added: string[]; removed: string[]; changed: string[] };
+
+// Compare a from-scratch build against the index that was already live. This exists to answer one
+// question with evidence rather than opinion: is the nightly full still FINDING anything, or is it
+// spending 40 minutes of disk rebuilding what the incremental had already got right?
+//
+// A run that reports no differences did no useful work. A run of those is the case for retiring the
+// nightly; a delta that keeps reappearing names the albums to go and look at.
+//
+// Compared field-by-field rather than by JSON.stringify, because two entries built by the same code
+// can serialise with different key order (the optional fields are spread conditionally) and that would
+// report drift where there is none.
+function diffManifest(prev: Manifest, next: Manifest): ManifestDelta {
+  const canon = (e: ManifestEntry) =>
+    [e.v, e.cover, e.tracks, e.videos ?? 0, e.images ?? 0, e.disco ?? false, e.lyrics ?? 0, e.posters ?? 0].join('|');
+
+  const added: string[] = [];
+  const changed: string[] = [];
+  for (const [rel, entry] of Object.entries(next.albums)) {
+    const before = prev.albums[rel];
+    if (!before) added.push(rel);
+    else if (canon(before) !== canon(entry)) changed.push(rel);
+  }
+  const removed = Object.keys(prev.albums).filter((rel) => !next.albums[rel]);
+  return { added, removed, changed };
+}
+
+function logManifestDelta(prev: Manifest, next: Manifest): void {
+  // A cache-format upgrade rebuilds every album by definition, so the delta is expected and says
+  // nothing about drift. Label it rather than let it read as 6k albums of rot.
+  if (prev.version !== next.version) {
+    console.log(`[music] full reindex: cache format v${prev.version} → v${next.version}, delta below is the upgrade itself`);
+  }
+
+  const { added, removed, changed } = diffManifest(prev, next);
+  const total = added.length + removed.length + changed.length;
+  if (total === 0) {
+    console.log('[music] full reindex delta: NONE — the incremental index was already identical');
+    return;
+  }
+
+  console.log(`[music] full reindex delta: +${added.length} added, -${removed.length} removed, ~${changed.length} changed`);
+  const sample = (label: string, rels: string[]) => {
+    for (const rel of rels.slice(0, 5)) console.log(`[music]   ${label} ${rel || '.'}`);
+    if (rels.length > 5) console.log(`[music]   ${label} …and ${rels.length - 5} more`);
+  };
+  sample('+', added);
+  sample('-', removed);
+  sample('~', changed);
+}
+
 /**
  * Full, from scratch: build a complete index into a FRESH slot without touching the live one, then swap
  * it in atomically only on success — a failed rebuild leaves the live index untouched. For the nightly cron.
@@ -652,6 +703,8 @@ export async function reindexNow(): Promise<IndexStatus> {
 export function reindexFull(): Promise<IndexStatus> {
   return withIndexLock(async () => {
     await ensureCacheSetup();
+    // Read the live index BEFORE the swap — it is what the delta below is measured against.
+    const live = await loadManifest();
     const slot = slotPath(String(Date.now()));
     await rm(slot, { recursive: true, force: true }).catch(() => {});
     // Empty prev ⇒ everything rebuilds into the fresh slot.
@@ -660,6 +713,7 @@ export function reindexFull(): Promise<IndexStatus> {
       await rm(slot, { recursive: true, force: true }).catch(() => {});
       console.error('[music] full reindex failed — live index left untouched');
     } else {
+      logManifestDelta(live, next);
       await activateSlot(slot);
       manifest = next;
       manifestLoaded = true;