Refuse every multi-track selected-tracks render, both scopes; make a failed mono collapse observable

src/shell/capture/capture.h

@@ -13,6 +13,7 @@
 #include "core/model/bank_model.h"
 #include "core/capture/capture_name.h"     // CaptureName — label + file-stem base
 #include "core/capture/render_settings.h"  // TailMode — the three-state tail contract
+#include "core/capture/wav_codec.h"        // MonoCollapseOutcome — the collapse's report
 
 // Forward-declared, never dereferenced here — only the REAPER-facing .cpp touches these.
 class MediaTrack;
@@ -85,7 +86,7 @@ enum class CaptureStatus {
     UnsupportedFormat,  // requested bit depth has no known REAPER blob (Float32 only)
     RenderFailed,       // the render action ran but produced no output file
     TransportBusy,      // realtime backend: transport already playing/recording — refused
-    MultiTrackRange,    // a ranged item capture whose items span >1 track — would render N files
+    MultiTrackSelection,// a selected-tracks render over >1 track — would render N files
     BoundsMismatch,     // the rendered file's frame count is not the requested window's
 };
 
@@ -126,9 +127,10 @@ CaptureName captureNameFor(const std::vector<std::string>& sourceNames,
 // bit-identical (the pure `collapseToMono` decides). Every other file is left
 // untouched, byte for byte, so the not-collapsed path is exactly what the backend
 // produced. Must run BEFORE stampCaptureSample, which measures the landed file.
-// Returns whether the file was actually rewritten (collapsed AND the write landed) —
-// callers use it to make the collapse observable in the reported CaptureResult.
-bool collapseCapturedFileToMono(const std::string& absolutePath);
+// A Failed outcome is ALSO logged to the console here, because a successful capture's
+// CaptureResult::message is not printed by any caller — the return value alone would
+// leave a genuine I/O failure indistinguishable from a legitimately stereo capture.
+MonoCollapseOutcome collapseCapturedFileToMono(const std::string& absolutePath);
 
 // Stamps the metadata shared by both backends onto `s`: trackGuids (echoed from the
 // request) + channelCount (measured from the produced file's `fmt`; 0/unknown as the