fix: musical content — 808 timing, chord voicings, melody range, pad arpeggiation, Ozone paths

- 808 bass: fixed note positions to beat 1.0 per bar (i-iv-i-V, 1.5 beat duration)
- Chords: 4-note 7th voicings (Am7, F7, C7, G7) instead of 2-note intervals
- Lead: constrained to 8-semitone range, pentatonic scale
- Pad: arpeggiated eighth-notes instead of static 2-note drones
- Ozone 12: fixed .vst3 filename paths in Calibrator
- Delta-encoding: fixed cumulative timing drift in _build_midi_source() with CC events

298/298 tests pass.

.sdd/changes/fix-musical-coherence/spec.md

@@ -0,0 +1,64 @@
+# Spec: Fix Musical Coherence
+
+## Requirements
+
+### R1: Bass Pattern
+- 808 Bass MUST use i-iv-i-V pattern over 8 bars
+- Each chord gets 2 bars with 1 note on beat 1.0, duration 1.5 beats
+- Pitch sequence for Am: A1(33), D2(38), A1(33), E2(40)
+- Transposed by key difference from Am
+
+### R2: Lead Melody
+- Lead notes MUST NOT exceed 12 semitones between consecutive notes
+- Melody MUST use chord tones on strong beats (1 and 3)
+- Scale-based passing tones allowed on weak beats
+- Octave range constrained to ±6 semitones from root
+
+### R3: Chord Voicings
+- ChordEngine MUST produce 4-note voicings (root, 3rd, 5th, 7th)
+- Use m7 for minor chords, 7 for major chords
+- Voice leading keeps movement ≤4 semitones per voice
+- First chord respects requested inversion
+
+### R4: Pad Movement
+- Pad MUST have rhythmic movement (arpeggiated eighth-notes)
+- Cycle through chord notes in ascending order
+- Volume: 0.55 (prevents masking)
+- Duration per note: 0.5 beats (eighth note)
+
+### R5: Ozone 12
+- Master chain PluginDef MUST have correct .vst3 path
+- Filename field MUST match PLUGIN_REGISTRY entry exactly
+- No fallback to empty string path
+
+## Scenarios
+
+### S1: Bass timing verification
+Given: 8-bar section, key Am, bpm=95
+When: build_bass_track runs
+Then: First 4 notes at start positions 0.0, 8.0, 16.0, 24.0 (beats)
+And: durations all 1.5
+And: pitches 33, 38, 33, 40
+
+### S2: Lead range constraint
+Given: 4-bar hook motif in Am
+When: build_motif(style="hook") runs
+Then: all(n.pitch for n in motif) within 12 semitones of tonic
+And: max pitch - min pitch ≤ 12
+
+### S3: Chord voicing size
+Given: Emotion "romantic", key "Am", 8 bars
+When: engine.progression(bars=8) runs
+Then: Each voicing has len() == 4
+And: Notes include root, 3rd, 5th, 7th intervals
+
+### S4: Pad arpeggiation
+Given: 8-bar section, Am key
+When: build_pad_track runs
+Then: Clip has >24 MIDI notes (arpeggiated, not 3 sustained)
+And: Each note duration ≤ 0.5 beats
+
+### S5: Ozone vst3 path
+Given: SongDefinition with master_plugins after Calibrator.apply()
+When: RPPBuilder writes master FXCHAIN
+Then: VST element filename field is "Ozone 12 Equalizer.vst3"