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)

assets/shaders/billboard.frag

@@ -8,6 +8,7 @@ in VS {
     vec3 pivot;
     vec2 local;
     float tint;
+    flat vec2 fx;  // alpha, flash
 } v;
 
 layout(binding = 0) uniform sampler2D uTex;
@@ -15,10 +16,19 @@ uniform int uPass;
 uniform float uEmissive;
 
 layout(location = 0) out vec4 fragColor;
+const float kFlashWhite = 4.0;  // linear HDR white the flash blends toward (blooms)
+
+// 4x4 ordered (Bayer) threshold in (0, 1): alpha 1 keeps every pixel, 0 none.
+float bayer4(vec2 p) {
+    ivec2 i = ivec2(p) & 3;
+    const float m[16] = float[](0, 8, 2, 10, 12, 4, 14, 6, 3, 11, 1, 9, 15, 7, 13, 5);
+    return (m[i.y * 4 + i.x] + 0.5) / 16.0;
+}
 
 void main() {
     vec4 c = sharpSample(uTex, v.uv);
     if (c.a < 0.5) discard;
+    if (v.fx.x < 1.0 && v.fx.x < bayer4(gl_FragCoord.xy)) discard;  // dithered fade, shadows included
     if (uPass == 1) return;
 
     vec3 up = vec3(0.0, 1.0, 0.0);
@@ -48,5 +58,6 @@ void main() {
         float d = length(v.pivot.xz - uBlob.xz) / uBlob.w;
         col *= 1.0 - 0.35 * (1.0 - smoothstep(0.3, 1.0, d)) * (1.0 - clamp(v.local.y / 0.6, 0.0, 1.0));
     }
+    col = mix(col, vec3(kFlashWhite), clamp(v.fx.y, 0.0, 1.0));  // hit / capture flash
     fragColor = vec4(col, 1.0);
 }