Convert all LOCAL-noon Date anchors (new Date(y, m, d, 12)) to UTC-explicit
anchors (new Date(Date.UTC(y, m-1, d))) in test.mjs and test-cjs.cjs.
Add UAQ default-engine round-trip regression suite (5 cases).
Extend FCNA round-trips; update vitest header comment.
README: add "Day boundaries and time zones" section explaining the UTC-day
contract, the correct pattern for zone-aware Luxon DateTimes, and ISO-string
parsing behaviour. Quick Start examples updated to use Date.UTC.
CHANGELOG: document inherited UTC-day fix under [Unreleased].
Lock-step dependency: requires hijri-core fix (commit 3419378,
branch fix/utc-day-boundary). Both packages release together per ADR-013.
Verified: TZ={UTC,America/New_York,Pacific/Auckland} × {test.mjs,
test-cjs.cjs, test-crossval.mjs, vitest} — all pass (88+26+120+15 tests).
