Replace age() helper with set_age_secs() on SrttCache that
binary-searches for the maximum subtractable duration. Prevents
panic on Windows (Instant starts at boot) while still producing
the oldest representable instant for correct decay calculations.
Also removes ephemeral test-ubuntu.sh from git.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>