PR #170 round-2 review: both guards stayed green when removed. Adds an ErrInvalidID sentinel, a plain-prefix-then-'..' escape case asserted on the sentinel, and a no-reader FIFO case with a timeout. Flip-verified: allowing '/' in the id tail and dropping O_NONBLOCK each turn the suite red. Refs #169 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>