Factor out parse_since_from(input, reference_ms) so callers can compute
relative durations against a fixed reference timestamp instead of always
using now(). The existing parse_since() now delegates to it with now_ms().
Enables testable and reproducible time-relative queries for features like
timeline --as-of and who --as-of.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>