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

tests/test_wav_codec.cpp

@@ -775,6 +775,24 @@ static void testCollapsePreservesQuietNaNBitPattern() {
     if (!pcm.empty()) CHECK(bitsFromFloat(pcm[0]) == kQuietNaNBits);
 }
 
+// The file-side collapse has three outcomes, and a genuine I/O failure once reported
+// identically to "the channels differ" — a stereo file landing under a plain Ok. The
+// report is where they must part: Declined stays silent (a capture with nothing to
+// collapse reads as it always did), and the other two say different things.
+static void testCollapseOutcomesReportDistinctly() {
+    const std::string declined  = monoCollapseSuffix(MonoCollapseOutcome::Declined);
+    const std::string collapsed = monoCollapseSuffix(MonoCollapseOutcome::Collapsed);
+    const std::string failed    = monoCollapseSuffix(MonoCollapseOutcome::Failed);
+
+    CHECK(declined.empty());
+    CHECK(!collapsed.empty());
+    CHECK(!failed.empty());
+    CHECK(collapsed != failed);
+    // The failure must read as a failure, not as a quieter success.
+    CHECK(failed.find("failed") != std::string::npos);
+    CHECK(collapsed.find("failed") == std::string::npos);
+}
+
 int main() {
     testParseCanonicalStereo();
     testParseMonoAndLeadingChunk();
@@ -810,6 +828,7 @@ int main() {
     testCollapseDeclinesOnUnparseableBytes();
     testCollapseChangesContentHash();
     testCollapsePreservesQuietNaNBitPattern();
+    testCollapseOutcomesReportDistinctly();
 
     if (g_fail == 0) std::printf("wav_codec: all tests passed\n");
     else             std::printf("wav_codec: %d CHECK(s) FAILED\n", g_fail);