Add exact writer epoch opens

[xav-db]

Summary

• let managed callers open a writer at an exact nonzero authority epoch
• reject lower and equal stale epochs through the manifest fencing boundary
• preserve last-writer-open behavior for unmanaged callers
• integrate exact epochs with the current 0.15 WAL writer initialization path
• cover exact claims and existing fencer behavior

Stack

Required by HelixDB slatedb#1018 and Hyperscale slatedb#260.

The dependency revision is c64286e.

[xav-db]

Validation update:

• Full locked all-feature SlateDB tests pass: 2,091 unit tests plus integration and doc-test suites (with only declared ignored tests).
• Required workspace Clippy with warnings denied and formatting checks pass.
• The Node binding build and 49/49 tests pass locally.
• The original hosted Node job became stuck inside node --test despite the local suite completing in under two seconds. I cancelled that anomalous run and reran only the cancelled dependency path; the hosted Node job then passed in 4m46s, including package-content checks.
• All PR checks are now green.