Avoid repeated full copies when staging hook-prebuilt wheels #1395

Summary

Avoid copying an unchanged hook-prebuilt wheel in full on every run while keeping fresh hook results authoritative.

Context

In PR #1393, a wheel returned by a download hook is copied into wheels_prebuilt through a temporary file and atomically replaces the staged file. This correctly handles a fresh wheel with the same filename, but repeated runs can copy a multi-GB wheel again. The hook owns its returned file, so the current code leaves that original in place. See the review discussion.

Acceptance Criteria

• An unchanged hook result can be reused without another full copy when its identity can be established safely.
• Changed contents with the same filename replace the staged wheel atomically; the mirror and later build use the new contents.
• Filename existence alone is not treated as proof that contents match. The optimization does not expose the mirror to later changes to the hook-owned file.
• The ownership and cleanup policy for the hook-returned file is documented.
• Focused tests cover unchanged and changed hook results, including replacement failure.

Definition of Done

• Code and focused tests are complete.
• File-scoped quality checks and CI pass.
• Documentation is updated if the hook contract changes.
• A PR is reviewed and merged.

Agent Notes

Preserve the current configured resolver download behavior and local mirror URL handling. Measure or demonstrate the avoided copy; leave the exact optimization method open for design.