feat(browser): error-triggered session replay (replay.onErrorSampleRate) - #1154

Confidence 4/5 · likely safe to merge
The reviewed delta is a two-line regex-state fix with a regression test; nothing else changed since the last review.
_{quality 100/100 · no findings · tests covered · risk medium}

Adds replay.onErrorSampleRate: unsampled sessions buffer the last minute of rrweb in memory and upload it, then stream, when an error is recorded. The delta since the last review is the error-filter regex fix, which is correct and tested.

• matches resets RegExp.lastIndex before test, so a g/y filter drops every matching error
• claimReplayMode rolls a persisted record / buffer / off per session
• startBufferedRecording holds the last two snapshot segments until drain() uploads them
• trigger() drains the buffer, streams the rest and re-announces the session as recorded

_{648a5ca · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple to ask about one.}