feat(node): add audio_root, gate Music enrichment on it like video_root

musicbay.md's original call — Music needs no root, tag reads are cheap so
just cover the whole shared tree — didn't hold up against a real messy
library: everything under every shared folder got mixed together with no
way to scope Music down to an actual music collection. This adds an
audio_root setting, symmetric to video_root in every respect: signed
operator op (audio_root/audio_root_ack, MNP bumped to 0.10), validated
against a real directory in the group's own roots before a signature is
even asked for, gates tag/cover enrichment exactly like video_root gates
ffprobe/TMDB (nothing runs until it's set, only files under it once it
is), and a set/change fires a one-off sweep of whatever the folder already
contains.

The old trigger — sweep everything the instant "music" joins enabled_apps
— is gone along with the root-less design it belonged to; setting
audio_root is now the trigger, mirroring set_video_root's
enrich_video_root_fn exactly.

Test coverage mirrors the video_root suite: policy (refuse before a
signature round trip, accept/store correctly) and the enrichment gate
itself (nothing without a root, only files under it, sweep on set).

1 files changed, 4 insertions, 0 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/protocol.py b/packages/meshbay-common/src/meshbay_common/protocol.py
index 8f29c38..24fea8f 100644
--- a/packages/meshbay-common/src/meshbay_common/protocol.py
+++ b/packages/meshbay-common/src/meshbay_common/protocol.py
@@ -119,6 +119,10 @@ class MNP:
     # TMDB poster or a MusicBrainz cover.
     AUDIO_TRANSCODE_REQ = "audio_transcode_req"          # client → node: transcode this file id
     AUDIO_TRANSCODE_RESP = "audio_transcode_resp"        # node → client: cache hash/size/mime
+    # Which folder is the Music app's entry point for this group — same
+    # shape as VIDEO_ROOT above.
+    AUDIO_ROOT            = "audio_root"                  # operator → node: which folder is the Music entry point
+    AUDIO_ROOT_ACK        = "audio_root_ack"
     # Device linking. A new device files a request bound to a code it displays;
     # an already-pinned device of the same account approves it. Neither the hub
     # nor the node can produce the countersignature.