Fix watcher stat mtime probing

[benawad]

Summary

• add a shared validated stat helper for watcher/guard/wake mtime and signature reads
• prefer BSD /usr/bin/stat on Darwin while rejecting nonnumeric GNU filesystem-stat output
• fix BSD fm_path_sig to use integer %m instead of float %Fm and add Darwin coverage

Validation

• no-mistakes run 01KWD0AC18D09ZSFGXFAWT21TJ passed review, test, document, and lint on head 4f6f939
• no-mistakes push to upstream failed with 403 for benawad on kunchenguid/firstmate, so this PR is opened from the authorized fork benawad/firstmate
• manual pre-gate validation also passed: bash -n bin/.sh tests/.sh; for test_script in tests/*.test.sh; do bash ""; done

Pipeline

Updates from git push no-mistakes