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/patch_plugin_path.py

@@ -0,0 +1,8 @@
+import io
+p = 'tools/test_vst2_host.py'
+s = io.open(p, encoding='utf-8', newline='').read()
+old = "PLUGIN = r'C:\\Program Files\\Steinberg\\VSTPlugins\\DUNE 3.dll'"
+assert old in s, 'not found'
+s = s.replace(old, "PLUGIN = r'C:\\Program Files\\Steinberg\\VSTPlugins\\WaveObserver.dll'")
+io.open(p, 'w', encoding='utf-8', newline='').write(s)
+print('patched')