Overview

Apply the primary's own oplog entries for opted-in writes using the existing OplogApplier machinery, and adjust write acknowledgment and read timestamp selection so read-your-writes behavior is preserved.

Scope of Work

• Run the batch OplogApplier on the primary, tailing from lastApplied; skip entries for non-opted-in writes (already applied).
• Separate lastWritten (oplog committed) from lastApplied (applied by the primary applier).
• Write concern (including w:1) waits for the primary's lastApplied to reach the write's optime.
• Read timestamps on the primary, including transaction snapshots and the detector read timestamp, are at or below lastApplied.
• Read-your-writes jstests.

Acceptance Criteria

• Opted-in writes are applied on the primary and are visible to subsequent reads after acknowledgment.
• Findings recorded on this ticket, including an audit list of every consumer of lastApplied on the primary (snapshot manager, stable timestamp, heartbeats, causal consistency, etc.).

Technical Notes

• Prototype quality; not intended for commit to master.

Design Note Alignment

• The primary applier runs under UnreplicatedWritesBlock, so its writes go through to WiredTiger rather than being staged, consistent with design note section 2.3.
• Design reference: src/mongo/db/txns/staged_writes_design.md (PR #66680).