Port Tier 1-2 GameDevJs example systems to engine

Extract and promote generic utilities and systems from GameDevJs2026 example
into main engine under pg namespace:

- Deduplicate setEntityVisibility → pg::setEntityVisibility (UI/utils.h)
- Promote XorShift32 → pg::LocalRng (Maths/randomnumbergenerator.h)
- Move GridAtlas → pg::GridAtlasLoader (Loaders/)
- Promote WorldFacts/FactChecker → Systems/factsystem.h/.cpp
- Move AutoSaveSystem → Systems/autosavesystem.h
- Move CameraSystem → 2D/camerasystem.h/.cpp, decoupled from grid.h
  (initial center now a constructor param) and InventoryUISystem
  (zoom suppression now a callback)

Update CMakeLists.txt source lists. Fix stale CameraSystem forward
declarations in placementoverlaysystem.h, spotlightoverlaysystem.h,
tileinspectorsystem.h, tutorialsystem.h that caused name collisions.

Author: Gallasko

examples/GameDevJs2026/Systems/placementoverlaysystem.h

@@ -9,7 +9,8 @@
 #include <cstdint>
 
 class HotbarSystem;
-class CameraSystem;
+namespace pg { class CameraSystem; }
+using pg::CameraSystem;
 
 // When the player has a building selected in the hotbar, tints the grid
 // cell(s) under the ghost preview green (placeable) or red (blocked).

examples/GameDevJs2026/UI/spotlightoverlaysystem.h

@@ -9,7 +9,8 @@
 
 #include "tutorialevents.h"
 
-class CameraSystem;
+namespace pg { class CameraSystem; }
+using pg::CameraSystem;
 
 // Imperative overlay used by the tutorial to focus the player on a single
 // target (a world tile, a UI element, or a fixed screen rect). Renders:

examples/GameDevJs2026/UI/tileinspectorsystem.h

@@ -11,7 +11,8 @@
 #include <cstdint>
 #include <string>
 
-class CameraSystem;
+namespace pg { class CameraSystem; }
+using pg::CameraSystem;
 class GridSystem;
 class HotbarSystem;
 class HudBarSystem;

examples/GameDevJs2026/UI/tutorialsystem.h

@@ -16,7 +16,8 @@
 #include "tutorialevents.h"
 
 class SpotlightOverlaySystem;
-class CameraSystem;
+namespace pg { class CameraSystem; }
+using pg::CameraSystem;
 
 // Forced early-game tutorial. State persists in WorldFacts as `tutorial_step`.
 // Each state advances on a specific event. The two ValidateMission* steps gate