Reuse Windows filesystem helper within apply_patch on 0.153.0

[hirsaeki]

Summary

Cleanly restack P1R on the stable rust-v0.153.0 baseline.

• reuse one lazily-created filesystem sandbox helper only within one logical Windows apply_patch filesystem phase
• bind reuse to the SandboxedFileSystem runner identity plus the full FileSystemSandboxContext
• keep Linux/macOS on the existing one-request-per-helper behavior
• keep the Windows Open / handle-transfer path one-shot
• invalidate the scoped helper on parent-side transport/framing failure
• propagate helper cleanup failure without misclassifying it as a sandbox denial
• retain the targeted Windows regression asserting one helper start per representative apply_patch

Base / patch shape

• base: 41e22fee981a63b3698df7ed36bad393cda24715 (rust-v0.153.0)
• one fork commit: ea1a73b351ad8da8fbfd421ac0bbb92da9999bee
• five changed files, all P1R production code or its targeted regression
• no P0 measurement code, P2 implementation, benchmark docs, generic helper pool, or CI workaround

The previous P1R base (c4ea7294b989827862eed6972d130233459161f4) and stable 0.153.0 diverge as release histories, so this was not rebased wholesale. The four existing files touched by P1R were compared across those bases and are unchanged; the reviewed final P1R blobs were therefore restacked directly onto the stable release tree without overwriting stable-specific source changes.

Validation

The fork-only repository-bundle workflow is no longer present on the stable-only main, and direct network access from the Sandbox is unavailable. No extra infrastructure patch was reintroduced just to run local checks. Repository CI on this PR is used for native build/test validation.