Ψ-W2-T2 remediation: atomic temp+rename collapse write, honest unknown-channel fallback, [verify — DAW] markers, corrected+filed bake-collapse deferral, observable collapse message, quiet-NaN test

src/core/capture/wav_codec.cpp

@@ -284,8 +284,13 @@ MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes) {
         }
     }
 
-    // float -> double -> float round-trips exactly (double represents every float),
-    // so channel 0 reaches the rebuilt file unaltered.
+    // float -> double -> float round-trips exactly for every finite value and for
+    // +-0/+-infinity (double represents every float bit pattern in those classes), so
+    // channel 0 reaches the rebuilt file unaltered. The one hole: a signaling NaN is
+    // quieted by the float->double promotion, so an identical-bit sNaN pair could
+    // collapse to a different bit pattern than it started with. Not reachable from
+    // REAPER-rendered audio, but the bit-identical predicate above admits NaN inputs,
+    // so this rebuild is not exempt from the claim it makes.
     std::vector<double> mono(frames);
     for (std::size_t f = 0; f < frames; ++f)
         mono[f] = static_cast<double>(pcm[f * stride]);