feat: overhaul MIDI demo — funky groove, multi-synth audio, DAW track view (#99)

Replace Bach arpeggios with an original "Groove in Em" composition: - 5 instruments: drums (kick/snare/hat/ghost/crash), bass, electric piano, lead synth, brass A (staccato stabs) + brass B (legato pads) - 96 BPM, E minor, 8-bar loop per commit (~20 s of audio each) - Multi-instrument Tone.js synthesis: MembraneSynth (kick), NoiseSynth (snare/ghost), MetalSynth (hi-hat/crash), MonoSynth (bass), PolySynth-FMSynth (electric piano), PolySynth-Synth (lead/brass)

5-act VCS narrative (13 commits, 4 branches): Act 1 Foundation · Act 2 Divergence (feat/groove + feat/harmony) Act 3 Clean Merge · Act 4 Conflict (cc_reverb on brass branches) Act 5 Resolution — all 21 MIDI dimensions active, v1.0 tag

Visual improvements: - DAW-style multi-track view (per-instrument rows with pitch mapping) - Fixed pause/resume (saves position, resumes from pausedAt) - Heatmap column highlight on commit select - Branch legend with color-coded blobs - Cleaner command log with ⚠/✓/$ line classification

All 1747 tests pass · 0 mypy errors · 0 typing_audit violations

Co-authored-by: Gabriel Cardona <gabriel@tellurstori.com>