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/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.