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)

docs/ASSET_GUIDE.md

@@ -104,7 +104,8 @@ so any documented endpoint parameter can be used.
 
 - **Feet on the lowest opaque row.** Billboards are planted by the lowest
   pixel with alpha ≥ 128 (`Texture::opaqueBounds`), so empty space below the
-  feet is fine but stray pixels below them make the sprite float.
+  feet is fine but stray pixels below them make the sprite float. Sheet
+  frames and atlas cells use a per-instance `anchor` instead (below).
 - **No baked ground shadows.** The engine casts real shadows and draws a
   contact blob under the player. PixelLab sometimes paints a grey ellipse under
   objects; regenerate or erase it (alpha 0) before use.
@@ -115,19 +116,55 @@ so any documented endpoint parameter can be used.
 
 - Sprite: `BillboardBatch b; b.loadTexture(path);` then push
   `BillboardInstance{pos, sway, size, flip, tint}` and `b.upload()`.
-  `sway` 0 = rigid, 1 = grass-like wind and player push.
+  `sway` 0 = rigid, 1 = grass-like wind and player push. The remaining
+  fields default to "whole texture, no effects": `uv` (sub-rect), `anchor`
+  (feet row v), `turn` (extra yaw, radians; paper-turn / facing hints, shadow
+  cards stay sun-facing), `alpha` (0..1 fade, dithered in color and shadow
+  passes; there is no blending) and `flash` (0..1 toward bright white for hit
+  or capture flashes).
 - Terrain layer: add the file stem to `kLayers` in `demo/DemoScene.cpp` (all
   layers must be the same size) and reference its index from a
   `TerrainMaterial`.
 
-### Characters and animation (not wired up yet)
+### Sprite sheets and atlases
+
+One instance draws one sub-rect of its batch's texture: `uv` = (minU, minV,
+maxU, maxV) in GL orientation (v = 0 is the image **bottom**), `size` = world
+size of that rect (`BillboardBatch::cellSize(cellPixels, 32)`), and `anchor`
+= v of the frame's feet row (`anchor < 0` falls back to the texture's lowest
+opaque row, which is wrong for any frame but the bottom row). `flip`
+mirrors inside the rect.
+
+- **Grid sheets** (PNG authored top-down, equal cells, no spacing):
+  `uv = Texture::cellUV(tex.size(), cell, col, rowFromTop)` and
+  `anchor = Texture::cellAnchor(tex.size(), cell, rowFromTop, feetRowFromCellTop)`,
+  where `feetRowFromCellTop` is the pixel row (0 = top of the cell) the feet
+  stand on. Keep the feet on the same row in every frame of an animation or
+  the sprite bobs. Leave at least one transparent pixel between a frame's
+  art and its cell edge, otherwise filtering (and minified mips) bleeds the
+  neighbour frame in.
+- **Runtime atlas** (`hd2d::SpriteAtlas`, for sprites created or downloaded
+  while the game runs): `atlas.init({64, 64}, cols, rows)`, `int slot =
+  atlas.add(image)` (bottom-centered in its cell, cropped to the cell, -1
+  when full), `set` / `remove` to replace or free a slot. Every cell has its
+  own 1 px transparent gutter. Share the page with
+  `batch.sharedTexture = &atlas.texture()` and use `uv = atlas.uv(slot)`,
+  `anchor = atlas.anchor(slot)` (the image's lowest opaque row). Images you
+  compose on the CPU should go through `bleedTransparentColors` (the atlas
+  and `loadImage` already do).
+- `Texture::create/load(..., nearest = true)` makes a `GL_NEAREST`, mip-less
+  copy for crisp ImGui previews; billboards should keep the default.
+- `hd2d_demo --sprite-test` shows a labelled row of atlas / sheet sprites
+  (flip, turn, alpha, flash, frame cycling) in front of the player.
+
+### Characters and animation
 
 PixelLab can make consistent characters with 4/8 directions and animations
 (`POST /create-character-v3`, `/characters/animations`, then
-`GET /characters/{id}/zip` or `/spritesheet`). To support them, add a new
-`kind` to `tools/pixellab.py` (submit, poll `GET /background-jobs/{id}` every
-5–10 s until `completed`, download) and a sprite-sheet animation path to
-`BillboardBatch` (UV rect per frame). Keep the 32 px/unit density.
+`GET /characters/{id}/zip` or `/spritesheet`). `tools/pixellab.py` does not
+download them yet: add a new `kind` (submit, poll `GET /background-jobs/{id}`
+every 5–10 s until `completed`, download) and play the frames through the
+sprite-sheet path above. Keep the 32 px/unit density.
 
 ## 4. High-resolution textures for 3D models
 
@@ -244,10 +281,19 @@ Always look at the result; don't assume it looks right.
 cmake -S . -B build -G Ninja && ninja -C build
 ./build/hd2d_demo --screenshot out.png --frames 90 --pos X,Z --dist 8 --yaw 30 --set dof=0
 ./build/hd2d_demo --screenshot rain.png --frames 90 --pos X,Z --rain 1
+./build/hd2d_demo --screenshot night.png --frames 90 --pos X,Z --hour 23   # also try --hour 6 / 18
+./build/hd2d_demo --screenshot ui.png --frames 420 --ui-demo   # game UI (hd2d::ui)
 ```
 
 - `--pos` puts the player (and camera target) at world X,Z; `--yaw` and
   `--dist` orbit the camera; `--set dof=0` removes blur for inspection.
 - Check every new model from at least two opposite yaws, in rain (wet look,
-  wind) and near water (reflection).
+  wind), at night (`--hour 23`: moonlight + fire light, emissives) and near
+  water (reflection).
 - Check sprites at default zoom next to the player for scale and pixel size.
+- UI fonts (`assets/fonts/`, loaded by `hd2d::ui::loadFonts`): OFL only, with
+  the font's `OFL.txt` saved as `<Family>-OFL.txt`; never modify or re-instance
+  a font with a Reserved Font Name (Cinzel). Dear ImGui's stb_truetype renders
+  a variable font's default instance, so choose fonts whose default weight
+  reads well. Check UI changes with `--ui-demo` at `--frames 120` (mid-reveal)
+  and `--frames 420`, at `--size 1920x1080` as well as the default size.