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.cpp

@@ -201,11 +201,27 @@ std::string makeUniqueTag(const std::string& prefix) {
            std::to_string(++counter);
 }
 
+void collapseCapturedFileToMono(const std::string& absolutePath) {
+    const std::vector<std::uint8_t> bytes = util::readFileBytes(absolutePath);
+    if (bytes.empty()) return;
+
+    const MonoCollapse collapse = collapseToMono(bytes);
+    if (!collapse.collapsed) return;
+
+    // One truncating write — the same shape, and the same accepted mid-write residual,
+    // as the realtime Auto-tail trim (capture_realtime_finalize.cpp).
+    std::ofstream out(absolutePath, std::ios::binary | std::ios::trunc);
+    if (!out) return;
+    out.write(reinterpret_cast<const char*>(collapse.bytes.data()),
+              static_cast<std::streamsize>(collapse.bytes.size()));
+}
+
 void stampCaptureSample(Sample& s, const CaptureRequest& req,
                         ReaProject* rateProj, ReaProject* timeSigProj,
                         const std::string& absolutePath) {
-    // Track GUIDs + channel count: echoed from the request (the caller resolved
-    // the selection; the backends stay source-agnostic).
+    // Track GUIDs echoed from the request (the caller resolved the selection; the
+    // backends stay source-agnostic). channelCount starts at the request value only
+    // as the fallback for an unparseable file — the produced FILE overrides it below.
     s.trackGuids   = req.trackGuids;
     s.channelCount = req.channelCount;
 
@@ -238,6 +254,10 @@ void stampCaptureSample(Sample& s, const CaptureRequest& req,
         const std::vector<std::uint8_t> fileBytes = util::readFileBytes(absolutePath);
         if (!fileBytes.empty()) {
             s.contentHash = hashWavContent(fileBytes);
+            // The one authority for the entry's channel count is the file's own `fmt`
+            // — never the render request, which asks for 2 on every capture path.
+            const WavLayout layout = parseWavLayout(fileBytes);
+            if (layout.valid) s.channelCount = static_cast<int>(layout.channelCount);
         }
     }
 
@@ -447,6 +467,14 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
         }
     }
 
+    // Lossless mono collapse, deliberately AFTER the bounds gate: the gate measures
+    // REAPER's own render against the requested window, so nothing of ours may sit
+    // between the render and that measurement, and a refusal must delete the
+    // renderer's file rather than one this step had already rewritten. The collapse
+    // preserves the frame count, so the two are order-independent in outcome — only
+    // in what each is measuring.
+    collapseCapturedFileToMono(expectedPath);
+
     // Record the request's own bounds (exact) rather than re-measuring the file.
     Sample s;
     // Same uniqueTag that named the file — calling makeUniqueTag() again could