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)

demo/DemoScene.h

@@ -1,6 +1,7 @@
 #pragma once
 #include "hd2d/Camera.h"
 #include "hd2d/Scene.h"
+#include "hd2d/SpriteAtlas.h"
 #include "hd2d/Weather.h"
 
 #include <array>
@@ -21,11 +22,24 @@ public:
     glm::vec3 playerPos{0.0f};
     hd2d::Weather weather;  // call weather.update() each frame before rendering
 
+    // Billboard feature test (`--sprite-test`; set before load()): a row of
+    // atlas / sprite-sheet sprites behind the player exercising sub-rect
+    // frames, flip, turn, alpha dither and flash. Labels are world-space
+    // points (sprite tops) for the caller to draw.
+    bool spriteTest = false;
+    struct Label {
+        glm::vec3 pos;
+        const char* text;
+    };
+    std::vector<Label> labels;
+
 private:
     struct Obstacle {
         glm::vec2 pos;
         float radius;
     };
+    bool buildSpriteTest(const std::string& assetDir);
+    void updateSpriteTest(float time, const hd2d::Camera& camera);
     void buildTerrain();
     void scatterProps();
     bool walkable(glm::vec2 from, glm::vec2 to) const;
@@ -45,6 +59,11 @@ private:
     std::mt19937 rng_{42};
     float playerSpeed_ = 4.2f;
     float walkPhase_ = 0.0f;
+    hd2d::SpriteAtlas atlas_;           // sprite test: runtime-packed cells
+    hd2d::BillboardBatch atlasSprites_;  // uses atlas_.texture() via sharedTexture
+    hd2d::BillboardBatch sheetSprites_;  // own texture: a 2x1 sheet read with Texture::cellUV
+    int atlasSlots_[3] = {-1, -1, -1};   // hero, flowers, tuft
+    int sheetFeetRows_[2] = {0, 0};      // per sheet cell, from the cell top
     float stepTimer_ = 0.0f;
     float facing_ = 0.0f;
 };