Stage24 keys-only — current source validation

PASS: new keys-only integration test: 231 key/style fixtures (three play styles, MIDI notes 24–100); single-note pitch/ownership, modifier suppression, BASS/performance/latch/voicing bypass, simultaneous notes, release, switching with held notes, settings v7 round trip and v1/v6 migration to Off.
PASS: complete selected new-feature regression runner, including Ambient/Stutter and previous Stage24 features, settings, backup, bootguard and looper.
PASS: dual-core regression runner, 5638 concurrent/serial comparisons, deadlines/fallback, stack guards, looper UI, 10368 voice policy fixtures, target C syntax with dual-core off/on.

Settings retain their 192-byte size. A retired scale byte stores the new option; older records migrate it to Off. No new audio buffer for this mode.
Protected QUAD, loader, boot-safety and golden files compared unchanged against preceding source: 24. No flash and no golden regeneration.
Still unknown: new target linking and precise flash/RAM totals, FM1 audio timing and hardware behavior. Existing VALIDATION-AMBIENT-STUTTER.txt describes the preceding effect changes.
