fix(node): the Music app's tags survive a restart

media_cache held video_meta, photo_meta and thumbs; the audio tags lived
only in the in-memory IndexEntry. So every start re-read every audio file
the node serves, and until that pass landed it served an index with no
artist on any track — one the Music app cannot group. Over a real
6176-file library the pass costs 27.8s cold and 6.4s from audio_meta.

Only what the bytes decided is stored. The filename and folder fallbacks
still run live, or a renamed file would get the old name's answer; the
sibling-cover scan reads the folder, so it stays live too; and a read
that failed is not cached, or one bad read becomes permanent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/CLAUDE.md b/CLAUDE.md
index 694f7b2..5463437 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -493,6 +493,24 @@ do. Read them before writing anything that touches the same mechanism.
   viewport from the layout one), which was plausible, cost a round trip, and
   was wrong; the screenshot showing no keyboard was already in hand.
 
+- **Derived data that nothing persists is re-derived where somebody is
+  watching.** `video_meta`, `photo_meta` and `thumbs` were durable; the audio
+  tags were not, so every node start re-read every audio file it serves and,
+  until that pass landed, served an index with no artist on any track — an
+  index the Music app cannot group. Nobody noticed for months because the
+  re-read is cheap and the node is usually up. `media_cache.audio_meta` holds
+  it now: the pass over a real 6176-file library costs 27.8s cold and 6.4s
+  cached. **What goes in such a cache is what the file's bytes decided, never
+  the field as the app finally sees it** — `display_title`, `track_no`, artist
+  and album can also come from the filename and the folder, and a rename has to
+  re-derive those; storing the resolved field would hand a renamed file the old
+  name's answer, which is the fault `_reenrich_renamed_audio_entries` exists to
+  prevent. The sibling-cover scan stays live for the mirror-image reason: it
+  reads the *folder*, so caching "no cover" would make a cover dropped in
+  afterwards unfindable. And a read that *failed* is not an answer — caching
+  "this file says nothing" for a drive that did not respond makes one bad read
+  permanent
+
 - **A view that draws one shape of unit, beside an emptiness test that counts
   every shape.** The Music grid's unit is an album; a track whose artist tag is
   empty belongs to none, so it was drawn nowhere — while `empty` counted it and