[Subscription] Reuse uncovered committed WAL retention boundary

[Caideyipi]

Summary

• Reuse the committed WAL retention boundary while committed writer progress advances monotonically, the boundary WAL file still exists, and its cached commit requirement remains uncovered.
• Recompute the boundary when those conditions change. The replay cursor continues to constrain WAL retention.
• Add regression coverage for boundary reuse, advancement, progress rollback, and replay cursor protection.

Motivation

In the reported partial-topic subscription workload, filtered writer entries repeatedly refreshed WAL retention. The CPU profile concentrated in WAL metadata reads during those refreshes. Reusing an unchanged, still uncovered boundary avoids reopening the same WAL file for each filtered entry.

Verification

• git diff --check passed.
• The focused regression test was not rerun after correcting its WAL entry helper because local tests were stopped at the requester's direction.