refactor(bridge): split fx bridge, stabilize VST3 host chain, drop old install engines

- remove diagnostic canaries from audio hot path (crackle fix) and
  stuck C5 note raw-MIDI test (Vst2Instrument::init noteOff)
- split native bridge into daw_vst_bridge (VSTi) + fx_vst_bridge (VST FX)
- VST3: processData prepare before setActive, zero input buses, accept
  both .vstpreset and bridge-state blob in RenderJob presets
- native_render/vst_engine: bridge-exe discovery + render_fx_chain + /fx-gui
- delete old install/ engine binaries (install via sidecar+resources now)
- add BUILD_LAYOUT.md, build_selfcheck, tools/ diagnostics

tools/test_midi_polyphony.js

@@ -0,0 +1,59 @@
+// tools/test_midi_polyphony.js — runnable check cho Phase A (dedup/legato/polyphony).
+// Đọc app.jsx, extract 2 hàm thuần, eval, assert spec §II.1 + §II.5.
+const fs = require('fs');
+const path = require('path');
+const src = fs.readFileSync(path.join(__dirname, '..', 'app', 'static', 'js', 'app.jsx'), 'utf8');
+
+const start = src.indexOf('const _cleanNativeNotes = (notes) => {');
+const end = src.indexOf('const _nativeTrackKey', start);
+if (start < 0 || end < 0) { console.error('FAIL: khong tim thay 2 ham'); process.exit(1); }
+const code = src.slice(start, end) +
+  ';globalThis._cleanNativeNotes=_cleanNativeNotes;' +
+  'globalThis._scaleNativePolyphony=_scaleNativePolyphony;';
+// eslint-disable-next-line no-eval
+(0, eval)(code); // indirect eval: const khai báo ở global scope
+
+let fails = 0;
+const assert = (cond, msg) => { if (!cond) { fails++; console.error('FAIL: ' + msg); } else { console.log('ok: ' + msg); } };
+const near = (a, b, eps) => Math.abs(a - b) < (eps || 1e-9);
+
+// 1. 8-note chord cùng lúc → tổng peak ≈ 0.3*8/sqrt(8) ≤ 1.0
+const chord = [];
+for (let i = 0; i < 8; i++) chord.push({ pitch: 60 + i, start_beat: 0, duration_beats: 2, velocity: 0.3 });
+const sc = _scaleNativePolyphony(_cleanNativeNotes(chord));
+const sum = sc.reduce((a, n) => a + n.velocity, 0);
+assert(sc.length === 8, 'chord giu 8 note');
+assert(near(sum, 8 * 0.3 / Math.sqrt(8)), 'tong peak ' + sum.toFixed(4) + ' ~= ' + (8 * 0.3 / Math.sqrt(8)).toFixed(4));
+assert(sum <= 1.0, 'tong peak <= 1.0');
+
+// 2. Solo → giữ nguyên velocity
+const solo = _scaleNativePolyphony([{ pitch: 60, start_beat: 0, duration_beats: 1, velocity: 0.8 }]);
+assert(near(solo[0].velocity, 0.8), 'solo velocity giu nguyen 0.8');
+
+// 3. Dedup: cùng pitch + start_beat → gộp, velocity max
+const dup = _cleanNativeNotes([
+  { pitch: 60, start_beat: 1, duration_beats: 2, velocity: 0.5 },
+  { pitch: 60, start_beat: 1, duration_beats: 3, velocity: 0.7 },
+]);
+assert(dup.length === 1, 'dedup gop 2 note');
+assert(near(dup[0].velocity, 0.7), 'dedup giu velocity cao 0.7');
+assert(near(dup[0].duration_beats, 3), 'dedup giu duration dai nhat');
+
+// 4. Legato: note trước cắt 0.01 beats trước note sau cùng pitch
+const leg = _cleanNativeNotes([
+  { pitch: 60, start_beat: 0, duration_beats: 4, velocity: 0.8 },
+  { pitch: 60, start_beat: 2, duration_beats: 1, velocity: 0.8 },
+]);
+assert(leg.length === 2, 'legato giu 2 note');
+assert(near(leg[0].duration_beats, 2 - 0.01, 1e-6), 'note dau cat con ' + leg[0].duration_beats);
+assert(near(leg[1].duration_beats, 1), 'note sau giu nguyen');
+
+// 5. Nối tiếp khác pitch → không cắt
+const seq = _cleanNativeNotes([
+  { pitch: 60, start_beat: 0, duration_beats: 4, velocity: 0.8 },
+  { pitch: 64, start_beat: 2, duration_beats: 1, velocity: 0.8 },
+]);
+assert(near(seq[0].duration_beats, 4), 'khac pitch khong bi cat');
+
+console.log(fails ? 'FAILED: ' + fails : 'ALL PASS');
+process.exit(fails ? 1 : 0);