Docs/UX: clarify compose vs vhs — stale rendered/*.mp4 causes freeze guard failures

[jmjava]

Problem

Users run docgen compose after editing a .tape but forget docgen vhs. Compose reuses terminal/rendered/*.mp4; freeze guard errors look like a tuning problem when the fix is re-rendering VHS.

Proposal

• Document prominently: always vhs → compose after tape edits.
• Optional: compose warns if .tape mtime is newer than matching rendered/*.mp4.
• Optional: --stale-check flag or config compose.warn_stale_vhs: true.

Related

FREEZE GUARD error: visual is Xs but audio is Ys → Z% frozen (max 25%)