Collapse a dual-mono resample bake to one channel, like every other capture

prepareLanding takes the shared collapse on the staged buffer before the hash
and the channel-count read, so hash, entry and file all derive from one buffer.
A true-stereo bake stays byte-identical.

src/shell/capture/CLAUDE.md

@@ -69,7 +69,7 @@ detail not covered there:
 - `render_isolation` (`shell/capture`) — the transient upstream silencing a ranged ITEM render needs, as a stack RAII guard alongside the two above: the selected-tracks source prints everything flowing INTO the track, so each direct folder child's `B_MAINSEND` and each of the track's receives' `B_MUTE` are cut for the render and restored on every exit path. Direct children only — a grandchild reaches the track through the child that owns it. The child-set walk is pure (`core/capture/track_topology`).
 - `capture_orchestrator` (`shell/capture`) — single-capture orchestration + the realtime/insert action bodies (Q-W3 hoist, T4-02): `renderOffline` (one offline render under the scope's FX-bypass guard), `captureAndIndexOne` (render + provenance stamp + bank add + tracking-ledger record, unpersisted), `RunCapture`/`RunCaptureItemAssign`, `RunCaptureRealtimeTrack`/`RunCancelRealtime` (the realtime action bodies — the in-flight state lives in `realtime_lifecycle`), and `RunInsertSelected` (the capture family's deliberate exception to capture-never-places — see the Invariants section above for `render_in_place`, the directory's other placing path, which sits outside the capture family entirely).
 - `bake_land` (`shell/capture`) — the EXTENSION's half of the resample chain, the SCAN PASS: scans every open project tab for pending `rsbake_*` requests, lands the ones belonging to the project this session has loaded (via `bake_landing`, below), and refuses the rest with `WrongProject` — one undo point for the batch, each answered over its own key inside the invoking instance's synchronous action call. It owns every ext-state read and write in the chain. The per-key verdict itself is NOT this TU's: it is `core/wire`'s pure `classifyBakeScan`, so this shell only enumerates, reads, and applies — counting every verdict into a `wire::BakeScanTally` as it goes, printing `wire::describeBakeKey` for EVERY enumerated key (the only thing that names which key is whose) plus `wire::describeBakeScan` whenever any key went unanswered or any answer's write was not confirmed, in one `ShowConsoleMsg`. It PROVES every write — answer or stale-clear — by reading the key back (`wire::extStateWriteLanded`, whose home is `core/wire/ext_state_read.h`); an answer that did not land is the one no-answer the tally alone cannot show. That proof is three-valued (`wire::BakeWriteProof`): a read-back that overflowed, or a throw AFTER the `SetProjExtState` call, reports Unknown; a throw BEFORE it reports Rejected, because the write is then known not to have been made. Each key is materialized before any answer is written, so no `SetProjExtState` in this action mutates a set the enumerator is still walking. Answers are held UNENCODED until after the pass's single persist, so a landing whose pass never got its persist through is answered as a failure rather than as an `Ok` no reload would honour — `wire::bakeLandingAfterPersist` is the ONE route to a `Banked` landing, and no path here (dedup included) may assign that word itself. The undo block is stack RAII (`UndoBlock`). Both loops are guarded: a throw in the scan still writes the answers already prepared, and a throw in the write-back loop still prints the lines already accumulated — no path through this action can end in a silent console. It RENDERS NOTHING — the instrument already did, through its own engine in its own process, which is what makes the baked audio the sound the user approved and what keeps the voice engine out of the extension's link graph.
-- `bake_landing` (`shell/capture`) — landing ONE bake request, split off `bake_land` on the one-request / whole-pass seam; touches no REAPER API at all. Non-mutating `prepareLanding` and mutating `commitLanding` sit under separate catches in `attemptLanding` — a throw before anything was written is a clean refusal, a throw after it is reported as possibly partial. Replace-vs-add comes from `tracking::resampleLanding`; a replace keeps the entry's id and slot and never deletes the superseded file. Hash-dedup applies on the add path only, before the disk write, matching `updateSampleInPlace`'s "an in-place refresh is not an insert" — and a dedup hit still rides the pass's persist, because the entry it points at may be one the same pass just added. A refused index withdraws the bytes this call had just written — the self-cleanup carve-out from prune's deletion authority, stated in `prune_fs.cpp`'s header. It never persists: the pass does that once for its whole batch, which is why no landing may report itself as banked.
+- `bake_landing` (`shell/capture`) — landing ONE bake request, split off `bake_land` on the one-request / whole-pass seam; touches no REAPER API at all. It takes the lossless mono collapse on the staged BUFFER (`wav_codec::applyMonoCollapse`, the same predicate the two backends' file-side `collapseCapturedFileToMono` runs) before the hash and before the channel-count read, so the hash, the entry and the written file all come from one buffer — a dead-center render lands 1-channel like any other dead-center capture. Non-mutating `prepareLanding` and mutating `commitLanding` sit under separate catches in `attemptLanding` — a throw before anything was written is a clean refusal, a throw after it is reported as possibly partial. Replace-vs-add comes from `tracking::resampleLanding`; a replace keeps the entry's id and slot and never deletes the superseded file. Hash-dedup applies on the add path only, before the disk write, matching `updateSampleInPlace`'s "an in-place refresh is not an insert" — and a dedup hit still rides the pass's persist, because the entry it points at may be one the same pass just added. A refused index withdraws the bytes this call had just written — the self-cleanup carve-out from prune's deletion authority, stated in `prune_fs.cpp`'s header. It never persists: the pass does that once for its whole batch, which is why no landing may report itself as banked.
 - `capture_batch` (`shell/capture`) — the batch-capture family + re-capture-from-source (Q-W3 hoist, T4-02): `RunBatchCaptureItems` (one sample per selected item), `RunBatchCaptureRazor` (one sample per razor area), `RunRecaptureFromSource` (regenerate a provenanced sample from its recorded source's current state, bank-only). Every unit routes through `capture_orchestrator` so every precision invariant holds; persist is batched to one ext-state write per action.
 - `realtime_lifecycle` (`shell/capture`) — the in-flight realtime-capture state machine + globals (Q-W3 hoist): the action starts it, `OnTimer` drives it per tick via `DriveRealtimeCapture` (a single-pointer-test idle fast path — load-bearing hot-path guardrail), `CommitRealtimeResult` lands a finished capture in the bank, `AbortRealtimeCaptureForUnload` tears down cleanly on extension unload.
 - `capture_realtime_shell` (`shell/capture`) — the async realtime-record backend surface (Q-W6 split of the former fat `capture.h`): `RealtimeRecordBackend::begin`/`tick`/`abort`, transport-driven across timer ticks (a realtime record cannot block REAPER's UI for its own duration). Deliberately shares NO interface with the offline backend — the lifecycles genuinely differ (the former `ICaptureBackend` interface was deleted in Q-W3, T4-26).

src/shell/capture/bake_landing.cpp

@@ -13,7 +13,7 @@
 #include <vector>
 
 #include "core/capture/capture_paths.h"   // deriveBankPaths
-#include "core/capture/wav_codec.h"       // parseWavLayout / hashWavContent
+#include "core/capture/wav_codec.h"       // applyMonoCollapse / hashWavContent
 #include "core/model/bank_book.h"
 #include "core/model/bank_model.h"
 #include "core/model/resample_name.h"     // the iteration-chain display name
@@ -85,7 +85,13 @@ PreparedLanding prepareLanding(ReaSamplerSession& session, const std::string& pr
                                   "the staged render was unreadable", request.generation);
         return prep;
     }
-    const WavLayout layout = parseWavLayout(prep.bytes);
+    // The lossless mono collapse, taken BEFORE the hash and before every measurement below,
+    // so the entry's channel count, its content hash, and the bytes commitLanding writes are
+    // all read off one buffer. A dead-center render is exactly the dual-mono shape the shared
+    // predicate folds; anything else comes back byte-for-byte as it was staged.
+    CollapsedWav staged = applyMonoCollapse(std::move(prep.bytes));
+    prep.bytes = std::move(staged.bytes);
+    const WavLayout layout = staged.layout;
     if (!layout.valid || layout.frameCount() == 0) {
         prep.settled = refuseBake(BakeStatus::StagedMissing,
                                   "the staged render is not a usable WAV", request.generation);

src/shell/capture/bake_landing.h

@@ -1,6 +1,7 @@
 #pragma once
-// bake_landing — landing ONE bake request into the loaded project's bank: read and hash the
-// staged WAV, resolve replace-vs-add, write the file, index it, seed its lineage. The scan
+// bake_landing — landing ONE bake request into the loaded project's bank: read the staged
+// WAV, collapse it losslessly to mono when it is dual-mono, hash it, resolve replace-vs-add,
+// write the file, index it, seed its lineage. The scan
 // pass that finds requests across the open tabs and answers them is `bake_land`; this is
 // what it calls per request, and it neither reads nor writes an ext-state key.