Current Stage24 fixes validation

PASS: actual emulator ISR audio test: nonzero dry synth output; Gate closes to zero; Off recovers; Stutter captures and holds audio after key release. Emulator mixer now calls the effect (previously only FM1 did).
PASS: every physical keyboard key in no chords, just keys, including the chord modifier and LOCK region; note-on/off and latch clearing. Option moved after Split Point. Existing 231 note/play-style fixtures and persistence migration checks pass.
PASS: complete tests/run_new_features.sh (exit 0), including all new effects, 71411 chord integration cases, 19890 CZ preset checks, 42553 overload protection checks, Room regression, eight-line Ambient stress/decay/model transitions, backup/settings/MIDI/bootguard/looper.
PASS: tests/run_dualcore_tests.sh (exit 0): 5638 PCM/state comparisons, timeout/fallback/stack guards, looper UI, 10368 policy fixtures and C syntax dual-core off/on.
Actual SELECT screen renders reviewed: docs/SELECT-PREVIEW.png.

Ambient changed to eight independently modulated delay lines with normalized Hadamard scattering and darker feedback damping. Existing delay buffers reused; small additional control state. Perceived metallicness is not proved by these tests: listen to Ambient in the emulator and FM1. Other reverb algorithms remain unchanged.
Unknown: current target flash/RAM totals and FM1 worst-case timing. Host comparisons do not measure hardware load. No device flash, QUAD changes or legacy golden regeneration.
Earlier VALIDATION files describe preceding revisions, not current subjective audio quality.
