Introduce a Calendar in Core that turns scaled clock time into whole
in-game days plus a fraction-of-day, with a configurable SecondsPerDay.
Pausing or changing TimeScale slows or stops it automatically. Pure
read-side and deterministic, registered via context.UseCalendar(...),
mirroring GameSpeed. Covered by xUnit tests; docs updated.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
