feat: professional reggaeton production engine — 7 SDD changes, 302 tests

- section-energy: track activity matrix + volume/velocity multipliers per section
- smart-chords: ChordEngine with voice leading, inversions, 4 emotion modes
- hook-melody: melody engine with hook/stabs/smooth styles, call-and-response
- mix-calibration: Calibrator module (LUFS volumes, HPF/LPF, stereo, sends, master)
- transitions-fx: FX track with risers/impacts/sweeps at section boundaries
- sidechain: MIDI CC11 bass ducking on kick hits via DrumLoopAnalyzer
- presets-pack: role-aware plugin presets (Serum/Decapitator/Omnisphere per role)

Full SDD pipeline (propose→spec→design→tasks→apply→verify) for all 7 changes.
302/302 tests passing.

.sdd/changes/smart-chords/proposal.md

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+# Proposal: Smart Chord Engine
+
+## Intent
+
+Current chord generation (`build_chords_track`) produces static root-position block chords with zero voice leading — every chord jump resets all 3 voices, producing audible jumps and amateur-sounding progressions. Add a `ChordEngine` class with voice leading, inversion selection, emotion modes, and genre-specific reggaeton progressions.
+
+## Scope
+
+### In Scope
+- New `src/composer/chords.py` with `ChordEngine` class
+- Voice leading: minimize semitone movement, max 4 semitone jump per voice
+- Inversion selection: root, first, second inversion
+- 4 emotion modes: romantic, dark, club, classic
+- Genre-specific reggaeton chord progressions per emotion
+- Deterministic: seed-based reproducibility
+- Modify `build_chords_track()` in `scripts/compose.py` to use `ChordEngine`
+
+### Out of Scope
+- Seventh/suspended/diminished chord types (use existing `CHORD_TYPES`)
+- Real-time chord generation (only batch/offline)
+- Other genres beyond reggaeton
+- Chord rhythm/pattern generation (only chord selection + voicing)
+
+## Capabilities
+
+### New Capabilities
+- `chord-engine`: `ChordEngine` class with seed-based deterministic progression generation, voice leading, and inversion selection
+
+### Modified Capabilities
+- `chords-track-generation`: `build_chords_track()` delegates to `ChordEngine` instead of hardcoded i-VI-III-VII
+
+## Approach
+
+**Pure Python, zero new dependencies** — all chord logic runs on MIDI note numbers using existing `NOTE_NAMES`, `SCALE_INTERVALS`, and `CHORD_TYPES` from `composer/__init__.py`.
+
+Voice leading: score candidate voicings by total semitone distance from previous chord; select lowest-score candidate within the 4-semitone max-jump constraint.
+
+Emotions → progression profiles:
+| Emotion  | Degrees | Quality flavor |
+|----------|---------|----------------|
+| romantic | i-VI-III-VII | softer, wider voicings |
+| dark     | i-iv-V-v  | minor-focused |
+| club     | i-VII-VI-V | driving, ascending |
+| classic  | i-VI-III-VII | tight block chords |
+
+## Affected Areas
+
+| Area | Impact | Description |
+|------|--------|-------------|
+| `src/composer/chords.py` | New | `ChordEngine` class |
+| `scripts/compose.py` | Modify | `build_chords_track()` uses `ChordEngine` |
+| `tests/test_chords.py` | New | Unit tests for voice leading, emotion modes, inversions |
+
+## Risks
+
+| Risk | Likelihood | Mitigation |
+|------|------------|------------|
+| Voice leading sounds worse than static | Low | 4-semitone cap prevents unnatural jumps; inversions smooth transitions |
+| Emotion modes too similar | Med | Each has distinct degree set and quality bias |
+
+## Rollback Plan
+
+Revert `build_chords_track()` to hardcoded progression. Delete `src/composer/chords.py`. One commit.
+
+## Dependencies
+
+None. Uses existing `composer/__init__.py` constants only.
+
+## Success Criteria
+
+- [ ] `ChordEngine(seed=42).progression(8)` returns identical output on repeated calls
+- [ ] No voice leap exceeds 4 semitones
+- [ ] All 4 emotion modes produce distinct chord sequences
+- [ ] `build_chords_track()` produces MIDI notes with `<4 semitone jumps between consecutive chords`
+- [ ] Existing tests pass unchanged