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/transitions-fx/proposal.md

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+# Proposal: transitions-fx
+
+## Intent
+
+9 sections play back-to-back with zero transition — the song feels like disjointed loops. Add transition FX (risers, impacts, sweeps) at section boundaries to glue sections into a coherent arrangement.
+
+## Scope
+
+### In Scope
+- Place transition FX clips (audio samples) on a dedicated "Transition FX" track at section boundaries
+- Riser/wash FX: 2–4 beats before section changes (e.g., build → chorus drop)
+- Impact/hit FX: on the downbeat of CHORUS, FINAL, VERSE2 entries
+- Filter sweep simulation via fade-in/fade-out on adjacent clips
+- Transition plan: which boundary gets which FX type + duration
+- Reuse existing FX-role samples from library (impacts, risers, transition FX, wash)
+
+### Out of Scope
+- Synthesized FX generation (numpy waveform synthesis) — deferred to future
+- MIDI CC filter automation in RPP (no CC support in builder today)
+- Per-track volume automation curves
+- Reverse cymbal (no suitable samples in library)
+
+## Capabilities
+
+### New Capabilities
+- `transition-fx`: Placement of audio FX clips at section boundaries for arrangement glue
+
+### Modified Capabilities
+None — existing section/track structure unchanged.
+
+## Approach
+
+**Audio samples from library** — the library has 57 FX-role samples including:
+- Impacts: `fx_C2_126_boomy` (2.5s, from `impact.wav`)
+- Risers: `fx_C#5_123_aggressive` (30s), `fx_G3_143_boomy` (6.6s, "RISER 3")
+- Transition FX: 4 "transicion fx" variants (1.0–1.7s)
+- Wash/noise: `fx_G#6_136_aggressive` (3.3s)
+- Short shots/gates: "CAMTAZO 1–2" (1.5–2.0s), "PUERTA" (0.2s)
+
+Place audio clips on a new "FX" track at section boundaries:
+- **Riser/wash**: starts 2–4 beats BEFORE boundary, ends on boundary downbeat
+- **Impact**: starts on boundary downbeat, short duration (1–2 beats)
+- Use existing `fade_in`/`fade_out` on ClipDef for filter-like sweeps
+- Use SampleSelector with `role="fx"` to pick compatible samples
+
+## Affected Areas
+
+| Area | Impact | Description |
+|------|--------|-------------|
+| `scripts/compose.py` | Modified | Add `build_fx_track()` — places FX clips between sections |
+| `src/core/schema.py` | Unchanged | `ClipDef` already has `position`, `length`, `audio_path`, `fade_in`, `fade_out` |
+| `src/reaper_builder/__init__.py` | Unchanged | Audio clip building already works |
+
+## Risks
+
+| Risk | Likelihood | Mitigation |
+|------|------------|------------|
+| FX samples may not match project key | Low | FX role is ATONAL_ROLES — key scoring skipped by selector |
+| Long riser samples exceed needed duration | Low | Clip length sets playback window; sample trimmed automatically |
+| No suitable riser for specific boundary | Med | Fall back to fade_in on next section clip |
+
+## Rollback Plan
+
+Remove `build_fx_track()` call from `main()`. Existing tracks untouched.
+
+## Dependencies
+
+- `data/sample_index.json` (already exists)
+- `SampleSelector` with `role="fx"` (already works)
+
+## Success Criteria
+
+- [ ] `python scripts/compose.py --bpm 99 --key Am` produces .rpp with FX clips at section boundaries
+- [ ] At least one FX clip between: build→chorus, chorus→verse2, bridge→final, outro end
+- [ ] FX clips have appropriate fade_in/fade_out curves
+- [ ] 110 existing tests continue to pass
+- [ ] Song renders without gaps — FX clips overlap/bridge sections