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

app/core/vst_engine.py

@@ -481,8 +481,8 @@ class PluginManager:
         """
         if not refresh and self._fx_scan_cache is not None:
             return self._fx_scan_cache
-        from app.core.native_render import find_bridge_exe
-        exe = find_bridge_exe()
+        from app.core.native_render import find_fx_bridge_exe
+        exe = find_fx_bridge_exe()
         results = []
         if exe:
             dirs = [d for d in [self.vst_dir] + self.extra_vst_dirs
@@ -629,7 +629,8 @@ def resolve_plugin_path(name_or_path: str, ext: str = "") -> str:
         dirs = list(_load_user_plugin_dirs()) or []
     except Exception:
         pass
-    dirs += [settings.VST_DIR, settings.SOUNDFONT_DIR]
+    from app.config import settings as _st
+    dirs += [_st.VST_DIR, _st.SOUNDFONT_DIR]
     for base_dir in dirs:
         if not os.path.isdir(base_dir):
             continue