Fix replay sequence reservation

[DorianOuvrard]

Summary

Fixes session replay sequence reservation so different replay payloads cannot upload with the same (sessionId, sequenceNumber).

Root Cause

The SDK previously advanced _sequenceNumber only after a successful fetch. A concurrent beforeunload/sendBeacon, reload, or oversized split could reuse the same sequence number for a different payload, especially seq=0 during replay startup.

Changes

• Reserve and persist replay sequence numbers before upload.
• Keep failed chunks as numbered retry payloads so exact retries reuse the same sequence number.
• Make sendBeacon use the same reservation path as normal flushes.
• Assign distinct sequence numbers to oversized split chunks.
• Guard concurrent startReplay() calls before rrweb finishes loading.
• Require the first chunk of a session to include a FullSnapshot.
• Add regression tests for initial flush races, beacon during fetch, retries, oversized split, and idle restart.

Validation

• npm run type-check
• npm test