Integrate loaded-track DJ metadata

[djdefi]

This wires the reviewed JAYDMETA/1 reader into DjSession and the index-v2 library lifecycle. Sidecars remain optional: missing, corrupt, unsupported, stale, or ambiguous metadata never blocks ordinary indexed AAC browsing or playback.

The integration keeps metadata association atomic with deck loads, publishes only compact copied snapshot fields plus semantic read-only accessors, invalidates associations on verified library identity changes, and prevents recursive index work while load/play/record activity is active. It does not add sync, quantize, seeking, loops, UI, or browser behavior, and it does not depend on JayD-Library CircuitMess#21.

Prerequisites and merge order

1. Fix Mixer clip() overflow bug; add per-channel hot-swap loading CircuitMess/JayD-Library#19 at 4ad5108ff1acc309ebc96d91d61d8cb24637f07e and MixScreen: load decks one at a time, stop blocking on both files CircuitMess/JayD-Firmware#42.
2. SongList: render visible tracks directly #2 (direct rendering), then Persist SongList index across opens #5 (index v2, final 307ba95ad9632d009b45f57ae821c59a53cfe617). This PR is stacked on Persist SongList index across opens #5.
3. Reviewed sibling foundations: Add host-side DJ metadata sidecar converter #1 (converter), Add firmware-owned DJ session authority #3 (DjSession authority), and Add bounded firmware metadata reader #4 (firmware reader). Their commits are preserved patch-equivalently in this branch; if those PRs land independently first, rebase this PR and drop only the duplicate dependency patches.
4. Merge this PR's focused integration commit ada60009a9abf06252ccfcd68cfb4ad482f59378 and review fix 8c0ae10acbb2e28a8ba303c29ac64a57763b8f52.

Dependency commit traceability

• DjSession: f5ade3d74d5e1541d145e9b14dcefa72ff9b0399 -> 7588cc1; follow-ups 41b1df3 -> c0a28bd, final 74c98d39a5872bc154fac72b628924911201f2e2 -> 6cdc72c
• Converter: faa9a2fd0cc957cf9ecb834c42295fc5ea91a3e5 -> f17d7aa; 6102f90 -> 2808d3b
• Firmware reader: 4e967e2 -> 68d25a8; final 68577ee2b1201e09a9f52519ab411e07eda1967c -> 5ea7fc8
• Index recovery follow-up: 307ba95ad9632d009b45f57ae821c59a53cfe617 -> 74f72e1

Each source/local pair has the same stable patch ID.

Behavior

• Opens bounded /library.jydm during SD/library initialization without making it a playback prerequisite.
• Resolves by normalized path plus available fingerprint/source identity and rejects stale or ambiguous matches.
• Publishes absent, valid, stale, corrupt, or unsupported, provenance/confidence, source rate/frame duration, BPM/key/rating, and cue/grid/downbeat/phrase capabilities and counts.
• Failed deck loads retain the prior deck and metadata. A successful Library Feature/hardware test CircuitMess/JayD-Firmware#19 openChannel() enqueue is the logical swap point; hot-swaps replace both association and path together.
• Verified SD/index identity changes invalidate loaded associations. Allocation failure during a gated reload preserves the last verified identity and attached deck metadata instead of publishing a transient zero identity.
• Index verification/rebuild stays deferred while either deck is loading, playing, or recording; the existing explicit Verifying/manual-rebuild policy remains bounded.

Validation

• python3 -m unittest -v tools/jayd-library/test_jayd_library.py: 13 passed
• python3 tools/jayd-library/firmware_reader_self_check.py: passed, including malformed and fractional-duration boundary fixtures
• CMake/CTest host self-checks: 3/3 passed
• Clean Arduino firmware build pinned to JayD-Library Feature/hardware test CircuitMess/JayD-Firmware#19 4ad5108: passed
• Firmware size: 1,791,510 bytes flash (49%), 47,192 bytes static RAM (14%)
• Review-fix delta from ada6000: +76 bytes flash, +0 bytes static RAM
• Delta from index-v2 base 5b36e506: +19,568 bytes flash, +40 bytes static RAM