docs(musicbay): note the audio_root amendment in the design doc

Records why the original no-root call was wrong (mixing, not cost) and
points at the new §4.3b protocol section — same treatment already given
to the WMA/Musepack transcode amendment above it.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/musicbay.md b/docs/musicbay.md
index 20e391e..2ca8b23 100644
--- a/docs/musicbay.md
+++ b/docs/musicbay.md
@@ -82,6 +82,16 @@ anywhere in this repo, the same discipline `mediacenter.md` §3.1 already
 follows for its own test corpus; a real accuracy figure belongs in an
 acceptance step (§8), not asserted here from a first look.
 
+**Amendment (2026-08-24):** an early version of this plan skipped a Videos-
+style scoping root, on the reasoning that tag reads are free/local (unlike
+ffprobe+ffmpeg thumbnailing) so the cost a root protects against was
+smaller. Real use showed the actual problem a root solves isn't cost, it's
+*mixing* — a shared tree with more than one kind of thing under it (a real
+messy library rarely holds only music) puts everything into one
+undifferentiated Music view with no way to scope it down. Music now has
+`audio_root`, symmetric to `video_root` in every respect including gating
+enrichment on it — see §4.3b.
+
 ### 2.2 No playback protocol change is needed at all — the big simplification
 
 Videos needed MSE, live ffmpeg remuxing, seek-by-restart, and a whole flow