refactor: migrate from FL Studio to REAPER with rpp library
Replace FL Studio binary .flp output with REAPER text-based .rpp output
using the rpp Python library (Perlence/rpp).
- Add core/schema.py: DAW-agnostic data types (SongDefinition, TrackDef,
ClipDef, MidiNote, PluginDef)
- Add reaper_builder/: RPP file generation via rpp.Element + headless
render via reaper.exe CLI
- Add composer/converters.py: bridge rhythm.py/melodic.py note dicts
to core.schema MidiNote objects
- Rewrite scripts/compose.py: real generator pipeline with --render flag
- Delete src/flp_builder/, src/scanner/, mcp/, flstudio-mcp/, old scripts
- Add 40 passing tests (schema, builder, converters, compose, render)
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