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/spec.md

@@ -0,0 +1,95 @@
+# Transition FX Specification
+
+## Purpose
+
+Glue sections together by placing audio FX clips at arrangement boundaries using existing `role="fx"` library samples.
+
+## Requirements
+
+### Requirement: FX Track Existence
+
+The system MUST create a dedicated "Transition FX" audio track with clips at 7 section boundaries.
+
+#### Scenario: FX track present in arrangement
+
+- GIVEN a 9-section song
+- WHEN `compose.py` runs
+- THEN a track named "Transition FX" exists with 7+ audio clips at boundary positions
+
+### Requirement: Riser Before Climax
+
+A riser/wash FX MUST start 2–4 beats before build→chorus and bridge→final boundaries, ending ON the boundary downbeat.
+
+#### Scenario: Riser before chorus
+
+- GIVEN build ends at beat 64 (bar 16)
+- WHEN FX is built
+- THEN a riser at position 60 (beat 60), length 4, `fade_in` ≥ 1.0s
+
+#### Scenario: Riser before final
+
+- GIVEN bridge ends at beat 176 (bar 44)
+- WHEN FX is built
+- THEN a riser at position 172, length 4, `fade_in` ≥ 1.0s
+
+#### Scenario: Riser before chorus2
+
+- GIVEN verse2 ends at beat 128 (bar 32)
+- WHEN FX is built
+- THEN a riser at position 124, length 4, `fade_in` ≥ 1.0s
+
+### Requirement: Impact on Section Downbeat
+
+An impact/stab FX MUST start on beat 1 of CHORUS (beat 64) and FINAL (beat 176).
+
+#### Scenario: Impact on chorus beat 1
+
+- GIVEN chorus starts at beat 64
+- WHEN FX is built
+- THEN an impact clip at position 64, length 1–2 beats, `fade_out` ≥ 0.2s
+
+#### Scenario: Impact on final beat 1
+
+- GIVEN final starts at beat 176
+- WHEN FX is built
+- THEN an impact clip at position 176, length 1–2 beats
+
+### Requirement: Transition Sweeps Between Verses
+
+Short transition FX MUST bridge chorus→verse2 (beat 96) and chorus2→bridge (beat 160).
+
+#### Scenario: Sweep bridges chorus to verse2
+
+- GIVEN chorus ends at beat 96
+- WHEN FX is built
+- THEN a transition clip at position 94, length 2 beats, `fade_in` and `fade_out` > 0
+
+### Requirement: FX Sample Selection
+
+The system SHALL select FX samples via `SampleSelector.select_one(role="fx")`, favoring short samples for impacts, long for risers.
+
+#### Scenario: FX role returns candidates
+
+- GIVEN 57 FX samples in library with ATONAL_ROLES including "fx"
+- WHEN `select(role="fx")` is called
+- THEN non-empty result returned; key scoring skipped (neutral 0.5)
+
+### Requirement: Fade Curves
+
+FX clips MUST use `fade_in`/`fade_out`. Risers: `fade_in` ≥ 0.3s. Impacts: `fade_out` ≥ 0.2s.
+
+#### Scenario: Riser fades in, impact fades out
+
+- GIVEN riser and impact clips defined
+- WHEN ClipDef is created
+- THEN riser.fade_in > 0 AND impact.fade_out > 0
+
+### Requirement: FX Track Mixing
+
+The FX track SHALL have volume ≤ 0.80 and send to Reverb/Delay returns.
+
+#### Scenario: FX track has moderate volume and sends
+
+- GIVEN "Transition FX" track created
+- WHEN track is defined
+- THEN volume = 0.72, send_level includes reverb (0.08) and delay (0.05)