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/design.md

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+# Design: Transitions FX
+
+## Technical Approach
+
+Add `build_fx_track()` to `scripts/compose.py` that places audio FX clips from the sample library at 7 section boundaries. Uses `SampleSelector.select_one(role="fx")` with per-type character hints. Reuses `ClipDef.fade_in/out`. New track inserted after Clap, before Pad — after main tracks, before sends are wired.
+
+## Architecture Decisions
+
+| Decision | Choice | Rejected | Rationale |
+|----------|--------|----------|-----------|
+| One FX track vs per-section | Single dedicated track | Per-section tracks | Simpler; one import per sample in REAPER; manageable clip count (7–9) |
+| Sample selection | Weighted random top-5 | Pinned specific files | Variety across runs; selector scoring already works |
+| Boundary timing | Hardcoded beat-offset map | Audio analysis | Section structure is deterministic; bar counts are fixed |
+| Riser+impact at chorus | Two clips, same boundary | Single combined clip | Requires different timing; riser before boundary, impact on it |
+
+## Data Flow
+
+```
+SECTIONS → offsets (bar → beat)
+         │
+         ▼
+FX_TRANSITIONS map: {boundary_idx: (type, start_offset, length, fade_in, fade_out)}
+         │
+         ▼
+build_fx_track(sections, offsets, selector, seed)
+  ├── for each entry in FX_TRANSITIONS:
+  │     ├── boundary_beat = offsets[boundary_idx] * 4
+  │     ├── position = boundary_beat + start_offset
+  │     ├── sample = selector.select_one(role="fx", seed=seed+idx)
+  │     └── ClipDef(position, length, audio_path, fade_in, fade_out)
+         │
+         ▼
+TrackDef("Transition FX", volume=0.72, clips=[...], send_level={...})
+```
+
+## Boundary → FX Map
+
+| # | Boundary | Beat | FX Type | Position | Length | Fade In | Fade Out |
+|---|----------|------|---------|----------|--------|---------|----------|
+| 2 | verse→build | 48 | sweep | 46 | 2 | 0.3 | 0.0 |
+| 3 | build→chorus | 64 | **riser** | 60 | 4 | 1.5 | 0.0 |
+| 3 | build→chorus | 64 | **impact** | 64 | 2 | 0.0 | 0.3 |
+| 4 | chorus→verse2 | 96 | transition | 94 | 2 | 0.2 | 0.2 |
+| 5 | verse2→chorus2 | 128 | riser | 124 | 4 | 1.0 | 0.0 |
+| 6 | chorus2→bridge | 160 | sweep | 158 | 2 | 0.2 | 0.2 |
+| 7 | bridge→final | 176 | riser | 172 | 4 | 1.0 | 0.0 |
+| 8 | final→outro | 208 | sweep | 206 | 2 | 0.3 | 0.5 |
+
+## File Changes
+
+| File | Action | Description |
+|------|--------|-------------|
+| `scripts/compose.py` | Modify | Add `FX_TRANSITIONS` dict + `build_fx_track()` (~50 lines); call in `main()` after clap track, before return tracks |
+
+## Key Implementation Detail
+
+`SampleSelector.select_one()` has a `seed` kwarg — new in the selector API. If not yet supported, use `select(role="fx", limit=5)` with manual `random.choice()`. Since FX is in `ATONAL_ROLES`, key compatibility scoring is skipped (neutral 0.5).
+
+## Track Ordering
+
+```
+tracks = [
+    build_drumloop_track(...),    # 0
+    build_perc_track(...),        # 1
+    build_bass_track(...),        # 2
+    build_chords_track(...),      # 3
+    build_lead_track(...),        # 4
+    build_clap_track(...),        # 5
+    build_fx_track(...),          # 6 ← NEW
+    build_pad_track(...),         # 7
+]
+return_tracks = create_return_tracks()  # 8 (Reverb), 9 (Delay)
+```
+
+Send wiring applies to all non-return tracks automatically via existing loop. FX track sends: Reverb=0.08, Delay=0.05.
+
+## Testing Strategy
+
+| Layer | What | How |
+|-------|------|-----|
+| Unit | `build_fx_track` returns TrackDef with 8 clips | Mock selector via `SampleSelector.__init__` patching |
+| Unit | Clip positions match boundary map | Assert `clip.position` values equal expected beats |
+| Integration | End-to-end .rpp output | `compose.py --bpm 99 --key Am --output test.rpp`; grep for "Transition FX" `<TRACK` block |
+| Existing | 110 tests pass | `pytest` before/after regression |
+
+## Open Questions
+
+None — all dependencies exist today (`SampleSelector`, `ClipDef.fade_in/out`, `SECTIONS` structure).