Refuse every multi-track selected-tracks render, both scopes; make a failed mono collapse observable

src/core/capture/wav_codec.h

@@ -117,6 +117,21 @@ struct MonoCollapse {
 // including the bext/source-position consequence beyond hashing.
 MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes);
 
+// How applying the collapse to a captured FILE ended. `Declined` is collapseToMono's own
+// "nothing to do"; `Failed` is a read that never happened or a warranted rewrite that did
+// not land. The capture is intact and correctly measured in every case — only the report
+// tells them apart, which is why the two must not share one value.
+enum class MonoCollapseOutcome {
+    Declined,
+    Collapsed,
+    Failed,
+};
+
+// The capture message's collapse suffix — empty for Declined, so a capture that had
+// nothing to collapse reads exactly as it did before the collapse existed. Shared by
+// both backends so one outcome cannot be reported two ways.
+std::string monoCollapseSuffix(MonoCollapseOutcome outcome);
+
 // --- Content identity (dedup hashes) -----------------------------------------
 
 // Deterministic FNV-1a 64-bit content hash over `len` bytes, as 16-char lowercase