Document shared checkpoint persistence

docs/internal/stage-transcribe.md

@@ -35,9 +35,10 @@ Generate raw per-speaker transcripts from prepared audio using WhisperX.
 - output path returned by adapter must match requested output path.
 - an empty adapter result path means the requested path; adapters cannot select
   an alternate destination.
-- candidate collection construction, manifest save, and installation into live
-  state are serialized. Expensive hashing and HTTP work remain outside that
-  critical section.
+- candidate collection construction, the shared
+  [checkpoint persistence boundary](manifest.md#checkpoint-persistence-boundary),
+  and installation into live state are serialized by transcribe's mutex.
+  Expensive hashing and HTTP work remain outside that critical section.
 - a canonical file has no completion authority until its manifest checkpoint
   succeeds. A save failure preserves the last successfully committed
   collection for ordinary retry.