Allow checkpoints for empty audio accepted by WhisperX

docs/internal/manifest.md

@@ -64,6 +64,8 @@ canonical `transcript_raw` output with its path, SHA-256 and size, and the
 producer run ID. Paths must be canonical and session-relative, checksums must
 be lowercase SHA-256 values, and the record and output producer identities
 must agree. Other stages cannot carry this transcribe-owned state.
+Audio size is nonnegative; WhisperX determines whether that audio can be
+transcribed. Output size remains positive and the transcript must be valid JSON.
 
 Transcribe reconciles this collection against the current prepared audio set
 and observable WhisperX semantics. Missing or corrupt regular outputs become