Ψ-W2-T2: collapse a capture whose channels are bit-identical to one lossless mono channel, index value measured off the landed file

src/shell/capture/capture.h

@@ -53,6 +53,9 @@ struct CaptureRequest {
 
     // 0 sampleRate => follow project rate.
     int         sampleRate   = 0;
+    // What the RENDER is asked for (RENDER_CHANNELS / the realtime record mode), not
+    // what the capture lands as: a dual-mono render is collapsed to 1 channel after
+    // the fact, and the Sample's count comes from the produced file.
     int         channelCount = 2;
     WavBitDepth bitDepth      = WavBitDepth::Float32;
 
@@ -100,8 +103,16 @@ public:
 // backend's family marker ("" offline, "rt-" realtime).
 std::string makeUniqueTag(const std::string& prefix);
 
-// Stamps the metadata shared by both backends onto `s`: trackGuids + channelCount
-// (echoed from the request), resolved sampleRate (request rate, else PROJECT_SRATE
+// Rewrites a just-captured WAV in place as a 1-channel file when its channels are
+// 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.
+void 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`; the request
+// value only as the fallback for a file that cannot be parsed), resolved
+// sampleRate (request rate, else PROJECT_SRATE
 // from `rateProj`), captureTempo, the capture-start time signature
 // (TimeMap_GetTimeSigAtTime against `timeSigProj` — offline passes nullptr for the
 // active project, realtime pins the record's own project), the WAV-aware