S7: stereo channel mode — core channel dimension, v4 mode state, VST3 bus negotiation, editor toggle

Per-instance mono|stereo (default mono, byte-identical). Stereo grows SampleData a 2nd
channel + a per-channel VoiceEngine render; decodeChannels applies the cross-mode policy;
setBusArrangements pins the mode's arrangement and restartComponent(kIoChanged) re-negotiates.

src/vst/reasampler_editor.h

@@ -113,6 +113,7 @@ private:
     std::vector<SampleChoice> visible_;   // samples_ narrowed by the active bank filter
     std::string selectedId_;              // the single-capture pick ("" = empty state)
     PerformanceMap map_;                  // the opt-in zones (empty = no zones)
+    ChannelMode channelMode_ = ChannelMode::Mono;  // S7 mono/stereo toggle snapshot
 
     // --- Transient UI state (not persisted; component state carries selection + zones) ---
     View view_ = View::kBrowser;          // default face is the browser