Fix cache expiration timestamps outside UTC

[cupkk]

PR Draft

Title: Fix cache expiration timestamps outside UTC

_get_mtime() creates an aware UTC datetime but passes its timetuple() to time.mktime(), which interprets the tuple in the host's local timezone. This shifts the configured cache expiration boundary on machines that do not run in UTC. For example, under America/Los_Angeles, the February 17, 2025 boundary is eight hours late.

Use datetime.timestamp() so the aware datetime is converted as UTC, and add a regression test that temporarily switches the process timezone before checking the timestamp. The test restores the original timezone in a finally block.

Local verification:

• reproduced the pre-fix 28,800-second offset under America/Los_Angeles
• ran the fixed function under the same timezone
• compiled the changed files and ran git diff --check
• verified the patch applies to a clean base snapshot

The full test suite and pre-commit hooks were not run because this minimal environment intentionally did not install the project's JAX/CUDA development stack. The change and test were prepared with AI assistance and manually inspected.

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