fix: preserve existing files when checking FSMap write access

[Bortlesboat]

Summary

Opening an FSMap with check=True can currently overwrite and delete an existing file named a; an a directory makes the check fail. Use a UUID-named write probe so the check leaves those existing entries intact and removes its own probe afterward.

Validation

• Four new regression cases fail on upstream and pass with the fix, covering files and directories on local and memory filesystems.
• Mapping, local and memory suites: 211 passed, 5 skipped, 11 xfailed on Python 3.13.12.
• Core tests excluding downstream: 537 passed, 148 skipped, 2 xfailed. The run emitted three Windows socket/thread teardown warnings and exited successfully.
• All applicable pre-commit hooks pass. Full Docker/FUSE and downstream integration suites were not run.

Post-Deploy Monitoring & Validation

For downstream adoption, check a populated mapper root with check=True and compare its keys and bytes before and after. Missing entries or leftover probe files are regression signals; callers can temporarily use check=False while investigating. The regression tests cover this check for this PR; live cloud validation remains with backend adopters.

Prepared and reviewed with Codex assistance. Review ran in the same agent session; no independent human review is claimed.

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