Sprite sheets, atlases, game UI, time of day, weather presets, headless captures

- BillboardInstance: per-instance uv sub-rect, feet anchor, turn, dithered alpha, flash;
  BillboardBatch::sharedTexture; Texture createEmpty/update/cellUV/cellAnchor/nearest
- SpriteAtlas: fixed-cell runtime atlas with gutters
- hd2d::ui: Octopath-style panels, fonts (Cinzel, EB Garamond), dialogue box with
  typewriter + portraits + choices, menus, nameplates, toasts, ImGui theme
- DayCycle (dawn/day/dusk/night keyframes, sun + moon arcs), Weather overcast/mist/
  lightning, night-aware presets, stars and moon in the sky
- Hidden-window offscreen captures (Window visible=false, Renderer output FBO,
  OffscreenTarget); --screenshot never opens a window
- Camera::worldToScreen; CMake usable via add_subdirectory (HD2D_BUILD_DEMO,
  HD2D_ENGINE_ASSET_DIR)

engine/include/hd2d/Particles.h

@@ -54,7 +54,8 @@ public:
     glm::vec3 fireLightColor{4.0f, 1.75f, 0.55f};
     float fireLightRadius = 7.0f;
     glm::vec3 wind{0.0f};       // world-space drift applied to smoke and embers
-    float ambientFade = 1.0f;   // scales fireflies + motes (Weather lowers it in rain)
+    float ambientFade = 1.0f;   // scales fireflies + motes (Weather lowers it in rain / mist)
+    float fireflyGlow = 1.0f;   // firefly brightness + light (WeatherLook::fireflyGlow, raised at night)
 
 private:
     struct Particle {