Parse a rate-limit reset value (X-RateLimit-Reset and similar) into the wait it names,
measured from now. Accepted forms: whole Unix epoch seconds (1760000000) and an ISO-8601
instant with T (or a space) and either Z or a ±HH:MM offset, fractional seconds
allowed (2026-10-18T10:00:00Z). A reset already past is Duration::ZERO; any other text
is None. A bare small integer is read as epoch seconds, so a delta-seconds value is not
accepted here; use parse_retry_after_at for those.
Parse a Retry-After header: delta-seconds or an HTTP-date (RFC 9110 section 10.2.3).
A date is turned into a delay from the current clock; a date in the past is zero.