Reject impossible /schedule calendar dates instead of overflowing the month

[cursor]

on YYYY-MM-DD only checked month 1–12 and day 1–31, so on 2027-02-31 / on 2026-06-31 / non-leap Feb 29 parsed as success. mktime then normalized the overflow (Feb 31 → early March) and the one-shot fired on a different civil day than the agent asked for.

parse_ymd now round-trips the date at noon and fails closed with invalid YYYY-MM-DD. Leap day on 2028-02-29 still works. Distinct from #318 (interval integer overflow) and #334 (resume/pause status).

unit_schedule_parser 8/8 locally (+ ASan/UBSan, 79 assertions).

merge-tree vs main: CLEAN. vs #334: schedule_parser.cpp / tests auto-merge (parse_ymd vs resume helpers); only CHANGELOG [Unreleased] sibling-conflicts, same as #321–#348.

  

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Note

Low Risk
Localized validation change in schedule phrase parsing with new unit tests; behavior tightens invalid input only and does not alter valid schedules beyond rejecting previously mis-normalized dates.

Overview
/schedule one-shot dates now fail closed on impossible calendar days instead of accepting on YYYY-MM-DD when only month/day ranges were checked and mktime silently rolled overflow (e.g. Feb 31 → March).

parse_ymd round-trips the parsed date through mktime at local noon and requires the normalized tm fields to match the input, which rejects Jun 31, non-leap Feb 29, etc., while still allowing valid leap days. Unit tests cover overflow rejection, leap-year acceptance, and the existing invalid YYYY-MM-DD error path.

The changelog [Unreleased] section documents this fix.

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