Keep completed interviewer captions readable

[chenyu50]

Completed Jim captions lose the opening sentences of long turns and disappear after 12 seconds while the candidate is still thinking. Keep the full completed turn visible until the candidate's next transcribed speech or an edit that changes the code text. Clicking or focusing the editor and switching languages preserve the caption, while candidate captions retain their existing 12-second timeout.

Mark a turn complete only after successfully reading its final transcription stream. Ignore late updates for retired Jim segment IDs and final replays of known, earlier candidate turns. Add behavioral regression tests, update the affected avatar assertions, and document the new caption rule in docs/avatar-contract.md.

Validation:

• All 1170 browser tests passed (0 skipped).
• In a local live interview, verified that a long Jim turn remained fully readable by scrolling, stayed visible beyond 13 seconds, and survived editor focus and caret movement.
• Verified that candidate speech replaced Jim's caption, a new Jim turn replaced the previous turn and remained visible, and a code text edit hid the caption.
• Added real-browser regression coverage for editor focus, clicks, caret movement, and language switches while a completed Jim caption is visible. Verified that a code text edit dismisses it.

Closes #223

---

Summary by cubic

Keeps completed interviewer captions visible in full until the candidate's next transcribed speech or a code text change. Previously, completed Jim turns were truncated to 160 characters and hid after 12 seconds while the candidate was still thinking, and editor interactions or language switches dismissed them.

• Completed turns no longer expire or truncate; candidate captions and the "Listening..." placeholder keep their 12-second timeout.
• A turn is only marked complete after its final transcription stream is read successfully; late updates for retired Jim turns and final replays of earlier candidate turns are ignored.
• An edit during the reveal is remembered and dismisses the caption once the final text finishes revealing.
• Editor clicks, focus, caret movement, and language switches preserve the caption; only changed code text or new speech dismisses it.
• Adds behavioral tests for retention, dismissal, pacing, idle timing, and scroll reset, and updates the avatar contract docs.

^{Written for commit 3ab26df. Summary will update on new commits.}

[jserv]

Rebase latest main branch and resolve conflicts.

[chenyu50]

Rebased onto main at 6d556b7 and pushed 954eb66. ./scripts/test.sh exited 0 with all 1058 browser tests passing and 0 skipped.

[jserv]

Rebased onto main at 6d556b7 and pushed 954eb66. ./scripts/test.sh exited 0 with all 1058 browser tests passing and 0 skipped.

Don't leave messages like the above, as GitHub always tracks commits.