feat(tracker): add sample offset and humanization

Implements two tracker realism features:

1. Sample Offset (compile-time):
   - Add offset_sec field to NoteParams and Sample structs
   - Parse OFFSET parameter in SAMPLE directive
   - Apply timing shift during compilation (zero runtime cost)
   - Use for attack-heavy samples to align perceived beat

2. Humanization (runtime, deterministic):
   - Add humanize_seed, timing_variation_pct, volume_variation_pct to TrackerScore
   - Parse HUMANIZE directive with SEED/TIMING/VOLUME params
   - Apply per-event RNG jitter using std::minstd_rand
   - Deterministic: same seed produces identical output

Both features work in real-time playback and WAV export.
Test file: data/test_humanize.track

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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