feat(mixer): live multi-color LUFS meters per deck

[robmorgan]

Summary

Replaces the flat green 6 px peak bars between the channel strips with 12 px segmented LED meters on a −40..0 LUFS scale, driven by the new MomentaryLoudness streaming BS.1770-4 meter from timestretch-rs 0.10.0 (robmorgan/timestretch-rs#26, merged).

• Audio callback (audio.rs): one MomentaryLoudness per deck (all 3, audition included) fed the post-trim pre-fader stereo pair per frame — the same tap as the peak meter. Published per block into a new DeckShared::meter_lufs atomic; no extra ballistics (the 400 ms window is its own integration). Meters reset on processor adoption and warm-start seeks so transitions don't ghost. The meter's push/read paths are allocation- and lock-free (verified against the ebur128 source), keeping the callback rules intact.
• Widget (app.rs): 3 px LED segments lit bottom-up, each colored by its own position — green below −14 LUFS, amber (ACCENT) to −9, red above — reusing the master/CPU meter palette. A light marker sits at the track's integrated LUFS (gain-match decks by aligning bars to markers), a red top strip flags peaks at the ceiling, and hover shows the numeric momentary/integrated values. Mixer panel widens 130 → 144 px.
• Worker (worker.rs): analysis now runs measure_loudness on the original interleaved signal (BS.1770 sums per-channel energies; the mono downmix reads ~3 dB low) and stores it in the artifact. Old rows deserialize with loudness: None — previously analyzed tracks just show no marker until re-analyzed.
• The marker reads pending_artifact before deck.pre_analysis, since a fresh analysis waits for a non-playing moment to reach the engine but the marker is display-only.

Testing

• 5 new unit tests in timestretch-rs, including an oracle test that streaming frame-by-frame matches a one-shot Mode::M measurement within 0.01 LU.
• cargo test --workspace (117 passed), cargo check --workspace --all-targets, cargo +nightly fmt --all.
• Manual: launched with a real track — ladder rises green→amber during loud sections, integrated marker rendered at the analyzed −9.7 LUFS, deck B (empty) shows a dark bar with no marker, no underruns in the log, layout intact at 144 px.

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