Add Calendar: in-game days layered over GameClock

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>

CLAUDE.md

@@ -28,7 +28,8 @@ is the living showcase — new engine features are demonstrated there.
 Engine libraries (each feature is a namespaced subfolder of its host):
 
 - **`Core`** — game loop, ECS world, scenes, time (`GameClock` with `TimeScale`; `GameSpeed`
-  for discrete pause/1×/3×/6× speed control over the clock, `context.UseGameSpeed(...)`),
+  for discrete pause/1×/3×/6× speed control over the clock, `context.UseGameSpeed(...)`;
+  `Calendar` turning scaled time into in-game days, `context.UseCalendar(secondsPerDay)`),
   plus **Input** (`MrGameEng.Input`: `InputManager`, `ActionMap`, `InputSystem`, in
   `Core/Input/`). Depends only on MonoGame and Friflo.Engine.ECS.
 - **`Graphics`** — custom batched renderer, camera, sprites, plus **Tilemaps**