Summary

Replace transaction-wide stable/ingest classification with per-operation mirror tracking and add prepared-transaction admission safeguards for async step-down.

Scope

• Add WT_TXN_OP_LAYERED_MIRRORED.
• Mark stable operations only after the matching ingest mirror succeeds.
• Reject unmirrored stable operations.
• Prevent unrelated raw ingest writes from masking an unmirrored stable operation.
• Remove txn->stepdown_ts_set after the replacement classification is in place.
• Add prepared-transaction admission and reservation tracking.
• Reject prepare and prepared-transaction claim during an active step-down window.
• Reject publishing a step-down boundary while prepared transactions are active.
• Release reservation accounting on all success and failure paths.
• Add timestamp validation, diagnostics, statistics, and invariant-focused tests.

Prepared transactions remain unsupported during an active step-down window.