Add MrGameEng.Mods: mod loading, JSON defs and localization

Mods are folders with About/About.json metadata; ModLoader resolves a
deterministic load order (dependencies first, ties alphabetical) and
later mods override earlier ones everywhere.

DefDatabase loads JSON def files ({ "type", "defs": [...] }) into
game-registered Def subclasses, with parent inheritance (own fields on
top of the parent's, nested values replaced whole), abstract parents
and full replacement of same-named defs by later mods.

LanguageManager loads Languages/<code>/*.json flat key-string maps,
switches language at runtime and falls back current -> default -> key.

ModContentTree merges one content folder across mods by relative path;
AtlasBuilder gains an explicit-sources Build overload so a merged
texture tree can be packed incrementally at game start.

The LittleSim game now ships its entire content as the Core mod,
demonstrating the module end to end.

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

CLAUDE.md

@@ -29,7 +29,10 @@ after `UseRenderer2D()`), `Pathfinding` (grid A*/Dijkstra/BFS and flow fields ov
 game-implemented `IPathGrid`; Core-only, owns no world data), `Collisions` (`Collider`
 component, spatial hash rebuilt per tick, pairs/queries/raycast; `scene.UseCollisions()`
 after movement systems), `UI` (Myra integration: `scene.UseUI()` after `UseRenderer2D()`),
-`DevConsole` (in-game console capturing `Core.Log`; `scene.UseDevConsole()` last in OnLoad).
+`DevConsole` (in-game console capturing `Core.Log`; `scene.UseDevConsole()` last in OnLoad),
+`Mods` (mod discovery + load order from `About/About.json`, JSON `Defs/` with parent
+inheritance and later-mod override, `Languages/<code>/` localization, merged content
+trees for textures; the game ships its own content as the `Core` mod).
 Dependency rule: every module may depend only on `Core`; `Core` depends only on
 MonoGame and Friflo.Engine.ECS. `Assets.Generator` is a netstandard2.0 analyzer.
 Documented exceptions: Myra renders with its own SpriteBatch internally; `Atlases`