Merge Ψ-W2-T2: a bit-identical capture collapses to one lossless mono channel

# Conflicts:
#	src/shell/capture/CLAUDE.md
#	src/shell/capture/capture.cpp
#	src/shell/capture/capture.h

src/shell/capture/capture.h

@@ -1,6 +1,7 @@
 #pragma once
 // The shared capture seam: CaptureRequest/CaptureResult (types both backends
-// speak), OfflineRenderBackend, and the makeUniqueTag/stampCaptureSample helpers.
+// speak), OfflineRenderBackend, and the helpers both backends share (naming, the
+// mono collapse, the Sample stamp).
 // Realtime's async begin/tick/abort surface lives in capture_realtime_shell.h.
 //
 // REAPER-free on purpose (bank_model only) so callers can depend on the seam
@@ -54,6 +55,9 @@ 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;
 
@@ -118,9 +122,19 @@ std::string makeUniqueTag(const std::string& prefix);
 CaptureName captureNameFor(const std::vector<std::string>& sourceNames,
                            int ordinal, const std::string& fallback);
 
-// Stamps the metadata shared by both backends onto `s`: trackGuids + channelCount
-// (echoed from the request), resolved sampleRate (request rate, else PROJECT_SRATE
-// from `rateProj`), captureTempo, the capture-start time signature
+// 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.
+// 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);
+
+// 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
+// fallback for a file that cannot be parsed — never the request's value, which is
+// always 2 and was never actually measured), 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
 // contentHash of `absolutePath` (left empty when unreadable), and createdTimestamp.