feat(browser): headless user feedback API - #1155

Confidence 5/5 · safe to merge
The reset of lastError in configureFeedback closes the shutdown case the previous review flagged, and the new browser tests exercise it.
_{quality 100/100 · no findings · tests covered · risk low · 1/1 new units observable}

Adds MapleBrowser.sendFeedback, a headless API that emits a maple.user_feedback OTel log event linked to the session, the last error's span and a replay it keeps. The earlier lastError-after-shutdown finding is fixed; the change is contained and safe to merge.

• sendFeedback emits maple.user_feedback with session.id, user.email/user.name and maple.feedback.* attributes
• configureFeedback is called from init() and from shutdown(), clearing lastError on either lifecycle start
• keepReplay runs before has_replay is read, so a buffered replay it keeps is reported

Fixed since the last review

• F2 · lastError survives shutdown() and links feedback to a dead trace

_{04e5138 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple to ask about one.}