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

native_bridge/src/Vst3Instrument.cpp

@@ -321,12 +321,6 @@ bool Vst3Instrument::loadPlugin(const std::string& path, double sampleRate) {
         std::cerr << "[Vst3Instrument] setupProcessing failed" << std::endl;
         return false;
     }
-    if (component->setActive(true) != kResultOk) {
-        std::cerr << "[Vst3Instrument] setActive failed" << std::endl;
-        return false;
-    }
-    processor->setProcessing(true);
-
     // Build per-bus buffers sized maxBlockSize_ (HostProcessData owns them;
     // we must NOT override channelBuffers with external pointers — unprepare
     // would delete[] them).
@@ -335,6 +329,13 @@ bool Vst3Instrument::loadPlugin(const std::string& path, double sampleRate) {
         return false;
     }
 
+
+    if (component->setActive(true) != kResultOk) {
+        std::cerr << "[Vst3Instrument] setActive failed" << std::endl;
+        return false;
+    }
+    processor->setProcessing(true);
+
     // Count output channels of bus 0 (mono plugins duplicate ch0 → both outs).
     int32 outChannels = 2;
     if (component->getBusCount(kAudio, kOutput) > 0) {
@@ -848,6 +849,14 @@ void Vst3Instrument::processAudioBlock(float* outputL, float* outputR, uint32_t
                     std::memset(ob.channelBuffers32[c], 0, numSamples * sizeof(float));
         }
     }
+    if (s->processData.numInputs > 0) {
+        for (int32 b = 0; b < s->processData.numInputs; ++b) {
+            const Steinberg::Vst::AudioBusBuffers& ib = s->processData.inputs[b];
+            for (int32 c = 0; c < ib.numChannels; ++c)
+                if (ib.channelBuffers32[c])
+                    std::memset(ib.channelBuffers32[c], 0, numSamples * sizeof(float));
+        }
+    }
     tresult pr = processor->process(s->processData);
     float mx = 0.f;
     if (pr >= 0 && s->processData.numOutputs > 0) {