Parse a messy date string into ISO 8601 + components

Best-effort parse of a human date string. Heuristic: reports whether the value was ambiguous (e.g. 03/04/2024) and which components it had to assume, with a sub-1.0 confidence on ambiguity. Absolute dates only: relative expressions — a weekday without an explicit day of month ('next tuesday', 'Friday June 2024') or 'tomorrow' — are rejected because the endpoint reads no clock and has no anchor date to resolve them against. An input missing its day of month ('3pm', 'June 2024') parses with the day as a placeholder at confidence 0.7; a missing year alone ('March 5') is the normal omission and keeps 0.9. A weekday that contradicts the resolved date ('Sun 15 Jun 2024' is a Saturday) sets weekday_conflict=true at confidence <= 0.5 — the explicit date wins, but the contradiction is flagged.