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

src/core/capture/wav_codec.cpp

@@ -300,6 +300,16 @@ MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes) {
     return out;
 }
 
+std::string monoCollapseSuffix(MonoCollapseOutcome outcome) {
+    switch (outcome) {
+        case MonoCollapseOutcome::Declined:  return {};
+        case MonoCollapseOutcome::Collapsed: return " (collapsed to mono)";
+        case MonoCollapseOutcome::Failed:
+            return " (mono collapse failed -- left as captured)";
+    }
+    return {};  // unreachable for a valid enum; claim nothing rather than a wrong outcome
+}
+
 std::string hashBytes(const std::uint8_t* data, std::size_t len) {
     // FNV-1a 64-bit: deterministic, no dependencies, adequate for dedup identity.
     std::uint64_t h = kFnvOffsetBasis;