Align faulthandler watchdog timeouts with CPython - #9775

Summary

• Accept timeout= in dump_traceback_later and signum= in register, matching CPython 3.15.
• Convert watchdog timeouts through the existing signed pytime helper after repeat/exit truth conversion. Preserve integer-protocol precedence, float-subclass handling, exception order, microsecond rounding, and native/C-long limits before file lookup.
• Keep the watchdog's original deadline across condition-variable notifications instead of treating any wake as an expired timeout.
• Add deterministic conversion probes and a notification regression test. Tiny positive timeout probes use an invalid descriptor and never start a watchdog; the successful keyword probe uses a long, immediately cancelled timeout.

This patch does not change signal callbacks or frame traversal. Existing watchdog max_threads, file-retention, and cancellation-joining gaps remain outside its scope. It adds no work to the VM execution loop.

Validation

• Fresh Linux release build passed.
• Affected time/signal snippets pass under both RustPython and CPython 3.15.1. The previous RustPython binary fails the new timeout-keyword regression.
• Workspace Rust tests: 1,535 passed, 20 ignored. C-API tests: 149 passed, 2 ignored. Both strict all-target Clippy runs passed.
• Full test_faulthandler, test_time, and test_signal passed: respectively 51/65/57 tests run and 11/19/25 skipped. The affected pytest snippet cases passed.
• Full native snippet run: 242 passed, 6 failed. The same six failures reproduce with the previous binary: four AF_UNIX restrictions, one chown EINVAL in this executor, and the default build's missing SQLite feature. CPython 3.15.1 also reproduces the AF_UNIX and chown restrictions. This is not a full snippet pass.
• Independent read-only review and normal commit hooks passed.
• The final commit has the tested source tree. Windows and 32-bit runtime validation remain for hosted CI; no cross-platform runtime pass is claimed.

Reference

CPython 3.15.1 faulthandler implementation and Clinic wrappers.

AI assistance: OpenAI Codex assisted implementation, review, and validation. Both commits include an Assisted-by trailer.