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_processor.h

@@ -88,6 +88,16 @@ public:
     Steinberg::tresult PLUGIN_API process(
         Steinberg::Vst::ProcessData& data) override;
 
+    // S7 channel-mode bus negotiation. The instrument has ONE canonical output arrangement
+    // determined by its per-instance channel mode (mono -> kMono, stereo -> kStereo). We
+    // accept the host's proposal only when it matches that arrangement; otherwise we reject
+    // (kResultFalse) but keep the mode's arrangement, so getBusArrangement / getBusInfo always
+    // report the mode's channel count and REAPER routes accordingly. A runtime mode change
+    // updates the output bus + calls restartComponent(kIoChanged) to trigger re-negotiation.
+    Steinberg::tresult PLUGIN_API setBusArrangements(
+        Steinberg::Vst::SpeakerArrangement* inputs, Steinberg::int32 numIns,
+        Steinberg::Vst::SpeakerArrangement* outputs, Steinberg::int32 numOuts) override;
+
     //--- from IEditController -----------------------------------------------
     // Hands the host our LICE IPlugView editor.
     Steinberg::IPlugView* PLUGIN_API createView(Steinberg::FIDString name) override;
@@ -128,7 +138,23 @@ public:
     PerformanceMap performanceMap();
     void setPerformanceMap(const PerformanceMap& map);
 
+    // The per-instance channel mode (S7, D-E: mono | stereo). Read/written on the UI thread
+    // (the editor toggle) and read off-thread by getState/reloadFromBank; guarded by
+    // channelModeMutex_. NEVER read on the audio thread — process() renders against the host's
+    // negotiated output channel count, and reloadFromBank bakes the mode into the decode.
+    ChannelMode channelMode();
+    // Sets the mode. When it CHANGES, updates the output bus arrangement (mono->kMono /
+    // stereo->kStereo) and asks the host to re-negotiate I/O via restartComponent(kIoChanged),
+    // then reloads the instrument so the next block decodes the new channel count. A no-op set
+    // (same mode) does neither. UI thread only.
+    void setChannelMode(ChannelMode mode);
+
 private:
+    // Apply `mode` to the output audio bus's SpeakerArrangement (kMono / kStereo). Called from
+    // initialize (topology) and setChannelMode (runtime change). Does NOT re-negotiate — the
+    // caller drives restartComponent when appropriate.
+    void applyOutputArrangement(ChannelMode mode);
+
     ReaperBridge bridge_;
 
     // --- The audio-thread handoff (S4 real-time discipline) -----------------
@@ -178,6 +204,12 @@ private:
     std::mutex performanceMutex_;
     PerformanceMap performanceMap_;
 
+    // The per-instance channel mode (S7). Off-thread only (UI + getState + reloadFromBank);
+    // guarded against a getState/editor race. Default Mono preserves pre-S7 behavior. NOT read
+    // on the audio thread — process renders against the host's negotiated output channel count.
+    std::mutex channelModeMutex_;
+    ChannelMode channelMode_ = ChannelMode::Mono;
+
     // Latched from setupProcessing so setActive/reload can size against it. Read
     // off-thread only.
     double sampleRate_ = 44100.0;