Ψ-W2-T2: collapse a capture whose channels are bit-identical to one lossless mono channel, index value measured off the landed file

src/core/capture/wav_codec.h

@@ -90,6 +90,32 @@ std::vector<std::uint8_t> buildFloat32Wav(int nch, std::uint32_t rate,
                                           std::size_t frameCount,
                                           const std::vector<double>& interleaved);
 
+// --- Lossless mono collapse ---------------------------------------------------
+
+// The outcome of the bit-identical mono collapse. `collapsed == false` means the
+// caller must leave the source file exactly as it is — it writes nothing.
+struct MonoCollapse {
+    bool collapsed = false;
+    std::vector<std::uint8_t> bytes;  // the rebuilt 1-channel WAV; empty unless collapsed
+};
+
+// Collapses a multi-channel float32 WAV to one channel when EVERY channel of EVERY
+// frame carries the identical float BIT PATTERN. Bit equality, never an epsilon and
+// never `==` on floats: +0.0/-0.0 and two NaNs with differing payloads are NOT
+// identical and are never folded. Frame count, sample rate and bit depth are
+// preserved — only the interleave stride changes — so the collapse cannot lose
+// information, and a lossy downmix (summing differing channels) is not something
+// this can express.
+//
+// Declines for: bytes that do not parse; a file already at one channel; a zero-frame
+// file (no frame of evidence to act on); any differing channel pair.
+//
+// The rebuild is a canonical minimal WAV, so non-audio chunks (a renderer's `bext`
+// timestamp, iXML, LIST) do not survive it. That much hashWavContent already skips —
+// but the collapse rewrites the `fmt ` body and the `data` payload too, which moves
+// the file's content identity; see this directory's CLAUDE.md for what that costs.
+MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes);
+
 // --- Content identity (dedup hashes) -----------------------------------------
 
 // Deterministic FNV-1a 64-bit content hash over `len` bytes, as 16-char lowercase