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

docs/PLAN.md

@@ -2661,7 +2661,11 @@ LOSSY fold; this collapse is lossless by predicate) but contradicted in text. Th
 amendment: channel count is preserved except that bit-identical channels may collapse
 losslessly to mono; a lossy fold remains forbidden. Noted in the amendment: the old text
 was already untrue in the other direction — a mono source renders at `RENDER_CHANNELS=2`
-today (`capture_orchestrator.cpp:227`, hardcoded and never measured).
+today (`capture_orchestrator.cpp:227`, hardcoded and never measured). `[verify — DAW]`
+"lossless" is proven at the file-bytes level; it is not the same claim as the null
+test's playback-chain property (whether REAPER sums a 1-channel item on a stereo track
+at the same unity gain as a dual-mono 2-channel item) — see the acceptance criteria's
+own `[verify — DAW]` on that bullet below.
 
 **Surface boundary — owns:** `core/capture/wav_codec` (the pure bit-identity predicate +
 collapse plan, with `wav_codec` unit tests), `shell/capture/capture.cpp` (the post-render

docs/TODO.md

@@ -577,3 +577,42 @@ select/move the neighbour, or capture at track scope instead.
 **Done looks like.** Nothing to do — recorded so a future reviewer does not read the
 non-isolation as an oversight and re-propose closing it against the recipe's stated
 tracks-and-range-only shape.
+
+## Resample-bake landings don't apply the lossless mono collapse to a dual-mono render
+
+**Context (surfaced by Ψ-W2-T2, mono-collapse).** The collapse (`collapseCapturedFileToMono`
+/ `core/capture/wav_codec::collapseToMono`) ships for every extension capture path —
+offline, realtime, batch, recapture — but not for `bake_land.cpp`'s `landOne`, the
+resample bake's landing function. A dead-center instrument render (the common case
+that motivated Ψ.6 in the first place) is exactly the dual-mono shape the predicate
+collapses, so an un-collapsed bake keeps paying for the second channel it doesn't need.
+
+**Not deferred for the reason once given.** `landOne` reads the staged file into `bytes`
+once (`bake_land.cpp:101`), parses its layout (`:105`), hashes it (`:126`), derives the
+channel count twice (`:131`, `:178`), and writes it (`:165`) — all from that same one
+buffer, so collapsing `bytes` right after the layout parse would keep the hash, the
+channel count, and the written file consistent by construction; there is no ordering
+hazard here to defer around.
+
+**The real reason.** `bake_land.cpp` is Phase Ξ's freshly-landed surface
+(Ξ-W2-T1, the resample bake chain) and another team is actively remediating it. Landing
+a mutation there now would cross tracks mid-remediation for no urgent gain — the mono
+propagation this item would add is a size win, not a correctness one.
+
+**A mono capture already propagates through the bake for free**, so this item is scoped
+to the dual-mono-*render* case only: `runBake` / `instrument_bake.cpp` already renders
+however many channels the dialed sound has, and `bake_render.cpp:38` reads
+`sample.channelCount()` off that render rather than hardcoding 2 — a mono-programmed
+sound already bakes to a mono file today, with no change needed.
+
+**Intended fix.** Once `bake_land.cpp` is quiet, call `collapseToMono` on the staged
+`bytes` in `landOne` right after the layout parse (`:105`) and before the hash (`:126`),
+matching the offline/realtime insertion point (post-parse, pre-identity-read).
+
+**Priority / risk.** Low — a size optimization on an already-correct path, not a
+precision-invariant gap; the bake's dual-mono case still lands as a valid (if larger)
+stereo file today.
+
+**Done looks like.** A dead-center instrument bake lands as a 1-channel file with
+`Sample::channelCount` matching, the same way an offline dead-center capture does; a
+true-stereo bake is byte-identical to today's output.