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/DayCycle.h

@@ -0,0 +1,44 @@
+#pragma once
+#include "hd2d/Weather.h"
+
+#include <glm/glm.hpp>
+#include <vector>
+
+namespace hd2d {
+
+// Time of day: turns an hour into a complete clear-weather WeatherLook by
+// smoothly interpolating tweakable keyframes, and moves the directional
+// light along a sun arc by day and a moon arc by night (so shadows always
+// work). Feed the result to Weather::setClearLook() every frame; the hour is
+// usually driven externally (e.g. by a game server).
+class DayCycle {
+public:
+    struct Keyframe {
+        float hour;        // [0, 24)
+        const char* name;  // for tuning UIs
+        WeatherLook look;  // sunAzimuth / sunElevation / daylight are ignored (computed from the hour)
+    };
+
+    DayCycle();  // default keys: late night, dawn, morning, noon, afternoon, dusk, night
+
+    WeatherLook look(float hour) const;  // any hour, wraps to [0, 24)
+    float daylight(float hour) const;    // 0 night .. 1 day (sun height above the horizon)
+    float night(float hour) const;       // 0 day .. 1 night (stars + moon visibility)
+    // Direction of the directional light: x = azimuth, y = elevation
+    // (radians, see Sun); the sun between sunrise and sunset, else the moon.
+    glm::vec2 lightAngles(float hour) const;
+    bool moonUp(float hour) const;
+
+    std::vector<Keyframe> keys;  // sorted by hour; interpolation wraps around midnight
+    float sunrise = 5.0f, sunset = 19.0f;
+    float sunAzimuth = -0.6f;       // azimuth at solar noon
+    float sunSweep = 2.6f;          // azimuth travelled from sunrise to sunset
+    float sunMaxElevation = 1.05f;
+    float moonAzimuth = 0.45f;      // azimuth at the middle of the night
+    float moonSweep = 1.6f;
+    float moonMaxElevation = 0.62f;
+    float horizonFade = 0.12f;      // light elevation (radians) over which it fades in
+    float minElevation = 0.05f;     // light never goes lower (keeps shadows stable)
+};
+
+}  // namespace hd2d