Overview

Wire the conflict detector into commit for staged writes, following design note section 7, so that write-write conflicts are decided by the detector rather than WiredTiger.

Scope of Work

• Commit ordering: take the WriteSet accumulated by the write helpers; call ConflictDetector::tryCommit(writeSet, readTs, reserveSlot); on conflict throw WriteConflictException (non-transactional writes retry, transactions abort); apply staged writes to WiredTiger at the reserved timestamp with overwrite=true cursors that bypass the staging hooks; commit the unit of work.
• Move oplog slot reservation for non-transactional writes into tryCommit.
• Forbid preparing a transaction that has staged writes.
• Measure critical-section cost.
• Contention jstests (e.g. adapted from the POC contention and sum-invariant tests).
• Note: applying staged writes at commit is a deliberate intermediate step that gives a working end-to-end checkpoint. A follow-on ticket in this epic removes it so only the oplog entry commits.

Acceptance Criteria

• Concurrent conflicting writes are correctly serialized; non-conflicting ones commit.
• Findings recorded on this ticket: every commit-path change required, and critical-section cost measurements.
• Acceptance bar: the full jstest suite passes with featureFlagStagedWrites enabled, except for specific tests excluded with a documented reason tied to prototype scope.

Technical Notes

• Prototype quality; not intended for commit to master.
• Design reference: src/mongo/db/txns/staged_writes_design.md (PR #66680).