Soundtrack: migrate the four effect-bearing compositions and test-effects-lib.py to the rewritten audioeffects (effects program Phase 6)

[bdbarnett]

Stakes

The effects program rewrites every audioeffects class from scratch, and its options, macros and patches may all change (Brad's D2: the soundtrack is our art and a playspace; it can be rewritten; it should not limit our work). When that lands, the four soundtrack directories that build effects stop rendering and bouncing until they are migrated:

Directory	How it builds effects
AureliaOverture	audioeffects.create(...) in script strings (composition.py)
VelvetCircuit	the same; the soundtrack's only user of the drive classes
Perihelion	fx_shimmer.py loaded through _read_effect
ShimmerLab	ShimmerHall / AirSpace by name through mpvst_effect_adapter

Automata, NeonMeridian (no effect inserts, by design) and PatchTest build none. tools/test-effects-lib.py also calls every class with legacy constructor keywords and is rewritten in the same step.

What to do, when the effects roadmap reaches Phase 6

1. Migrate the four compositions to the new surfaces. A piece may be re-voiced rather than forced to sound as it did — there is no baseline (D4).
2. Rebuild the sidecar engine on the pinned audioif, both ports, clear a stale MPVST_AUDIOIF_LIB cache entry if present, rebuild and ctest, reinstall the plug-in (full build), and confirm the installed engine imports synthio with audioeffects/ staged beside it. The staged lib goes stale silently.
3. Render every piece offline and bounce it; run reaper/verify_song.py per piece (peaks, section levels, ordering, dead air).
4. Re-capture the render reference as a new reference: audioif/tests/parity/capture_render_reference.py --capture --python micropython-vst3/.venv/bin/python. D4 is the approval that re-capture needs.
5. Rewrite tools/test-effects-lib.py against the new surfaces; ctests green.

Plan: workspace docs/effects-roadmap.md, Phase 6. Trigger: the effects program closes Phase 5.