Ψ-W2-T2 remediation: atomic temp+rename collapse write, honest unknown-channel fallback, [verify — DAW] markers, corrected+filed bake-collapse deferral, observable collapse message, quiet-NaN test

src/shell/capture/capture.h

@@ -107,11 +107,14 @@ std::string makeUniqueTag(const std::string& prefix);
 // 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);
+// 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`; the request
-// value only as the fallback for a file that cannot be parsed), resolved
+// 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