Split the platform out of Core: new Host library + HeadlessHost

Core now depends only on Friflo.Engine.ECS — no MonoGame, no platform.
The windowed MonoGame host moves to the new MrGameEng.Host library:
GameHost, GameHostOptions, visual scene transitions (OverlayTransition,
Transitions.Fade/Wipe, TransitionRenderer) and the Input feature
(namespace MrGameEng.Input is unchanged). Transition timing stays in
Core (SceneManager exposes ActiveTransition/TransitionCoverage/
TransitionPhase; the host draws the overlay). EngineContext loses its
GraphicsDevice property: hosts publish the device as a service and
graphics code reads it via context.GetGraphicsDevice() in Graphics.

Core gains HeadlessHost: a fixed-timestep loop without a window or GPU
(Tick/RunTicks, Run with wall-clock pacing and lag resync) for dedicated
servers, batch simulation and tests. Graphics and Audio now carry their
own MonoGame.Framework.DesktopGL reference instead of inheriting it
from Core.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -27,13 +27,18 @@ 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(...)`;
-  `Calendar` turning scaled time into in-game days, `context.UseCalendar(secondsPerDay)`;
-  `Climate` — continuous seasonal/daily temperature and season over the calendar,
-  `context.UseClimate(settings)`),
-  plus **Input** (`MrGameEng.Input`: `InputManager`, `ActionMap`, `InputSystem`, in
-  `Core/Input/`). Depends only on MonoGame and Friflo.Engine.ECS.
+- **`Core`** — the platform-free kernel: ECS world, scenes and transition timing, time
+  (`GameClock` with `TimeScale`; `GameSpeed` for discrete pause/1×/3×/6× speed control over
+  the clock, `context.UseGameSpeed(...)`; `Calendar` turning scaled time into in-game days,
+  `context.UseCalendar(secondsPerDay)`; `Climate` — continuous seasonal/daily temperature
+  and season over the calendar, `context.UseClimate(settings)`), services, logging, and
+  **`HeadlessHost`** — a fixed-timestep loop without a window or GPU (dedicated servers,
+  batch simulation, tests). Depends only on Friflo.Engine.ECS — no MonoGame, no platform.
+- **`Host`** — the windowed MonoGame host: `GameHost` (wraps `Game`, owns the window and
+  `GraphicsDeviceManager`, publishes `GraphicsDevice` as a service), visual scene
+  transitions (`OverlayTransition`, `Transitions.Fade/Wipe`, `TransitionRenderer`), plus
+  **Input** (`MrGameEng.Input`: `InputManager`, `ActionMap`, `InputSystem`, in
+  `Host/Input/`). Nothing depends on `Host` except the game itself. → `Core`.
 - **`Graphics`** — custom batched renderer, camera, sprites, plus **Tilemaps**
   (`MrGameEng.Tilemaps`: code-built tile grids rendered through the batcher,
   `scene.UseTilemaps()` after `UseRenderer2D()`, in `Graphics/Tilemaps/`) and **Lighting**
@@ -70,9 +75,13 @@ Engine libraries (each feature is a namespaced subfolder of its host):
   netstandard2.0 analyzer.
 
 Dependency rule: a library may depend only on `Core` and `Graphics`; `Core` depends only
-on MonoGame and Friflo.Engine.ECS. Features grouped into one library share its package
-set (e.g. `Content` carries both FontStash and StbImage) — keep optional/heavy deps
-(Myra, NVorbis) in their own library so the rest of the engine stays free of them.
+on Friflo.Engine.ECS (no MonoGame — the simulation must run headless). MonoGame is pulled
+in by the platform/graphics libraries (`Host`, `Graphics`, `Audio`). Platform resources
+(e.g. `GraphicsDevice`) are published by hosts as services in `EngineContext.Services`;
+graphics code reaches the device via `context.GetGraphicsDevice()` (extension in
+`Graphics`). Features grouped into one library share its package set (e.g. `Content`
+carries both FontStash and StbImage) — keep optional/heavy deps (Myra, NVorbis) in their
+own library so the rest of the engine stays free of them.
 
 ## Commands