fix(startup): distinguish unrecognized composer boundaries (#634)

When OpenRig can't verify that a startup prompt was submitted, the warning now says why. "Unrecognized composer boundary" means the capture was available but the composer's marker or closing rule wasn't found. The existing extracted-text-mismatch wording is kept for real mismatches. A text-free reason label is added to the startup diagnostics.

- Applies at the initial and the after-retry observation.
- Wording and diagnostics only: the composer classifier, transport, guarded-Enter matching, verdicts, waits, retries and captures are unchanged.
- No prompt or reply text in the warning or the event fields.

Recording footer-shape metadata for unrecognized layouts is a possible follow-up, not part of this change.