fix(storage): evict construction cache at durable boundaries

[DecisionNerd]

Durable construction can exhaust its memory envelope through file-backed cache. Shared filesystem readers and writers now bound active cache windows, synchronize writes before Linux cache advice, and carry measured synchronization evidence through construction and import receipts.

Closes #1110. Refs #951 and #952. #951 retains the full-lifecycle attribution and S20/conditional S22 host outcomes.

The integration fixes three verified defects: empty reads incorrectly marked unread input as exhausted; CAS payload copies discarded periodic synchronization counts; and Windows direct-file certification attempted to synchronize a read-only handle. Boundary tests preserve exact synchronization counts, bytes, hashes, reopened content, and cleanup. Existing Windows CI now exercises the direct-file regression. A reproduced ETXTBSY race in test shell fixtures is fixed by invoking the shell explicitly.

Validation on OVHC-AGENCY:

• Filesystem: 31 tests passed; storage: 946 passed, 2 unchanged ignores; import: 14 passed; actual 1x/2x/4x scale lifecycle: 23 passed.
• Benchmark certification: 24 passed; generator: 2 passed; progressive qualification: 17 passed.
• Boundary regressions failed before the fixes and passed afterward. Native strace confirmed payload synchronization counts 1/1/2 below/at/above each tested window, before successful cache advice. Linux tiny-lifecycle receipt counts reconcile exactly to 38/63/113.
• Workspace and benchmark all-targets clippy, generator identity, formatting, and fast checks passed. Storage tests used an isolated ext4 temporary root as ubuntu.
• Independent review found no remaining issues. Exact-head CI Gate, native Windows regression, Rust, bindings, durability, concurrency, and real tiny lifecycle checks passed at c34ae921. Squash-merged as 581b44a2; fix(storage): reconcile bounded cache eviction synchronization #1110 is closed.

No host-scale certification outcome is claimed by this implementation PR.

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