Collapse a dual-mono resample bake to one channel, like every other capture

prepareLanding takes the shared collapse on the staged buffer before the hash
and the channel-count read, so hash, entry and file all derive from one buffer.
A true-stereo bake stays byte-identical.

src/core/capture/wav_codec.cpp

@@ -6,6 +6,7 @@
 
 #include <cstdio>   // std::snprintf (hash hex render)
 #include <cstring>  // std::memcpy, std::memcmp
+#include <utility>  // std::move
 
 namespace reasampler::capture {
 
@@ -300,6 +301,23 @@ MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes) {
     return out;
 }
 
+CollapsedWav applyMonoCollapse(std::vector<std::uint8_t> bytes) {
+    CollapsedWav out;
+    MonoCollapse collapse = collapseToMono(bytes);
+    if (collapse.collapsed) {
+        const WavLayout rebuilt = parseWavLayout(collapse.bytes);
+        if (rebuilt.valid) {
+            out.bytes = std::move(collapse.bytes);
+            out.layout = rebuilt;
+            out.collapsed = true;
+            return out;
+        }
+    }
+    out.bytes = std::move(bytes);
+    out.layout = parseWavLayout(out.bytes);
+    return out;
+}
+
 std::string monoCollapseSuffix(MonoCollapseOutcome outcome) {
     switch (outcome) {
         case MonoCollapseOutcome::Declined:  return {};