feat(assets): #81 namespaced spritesheet/atlas API (Sprite.* / Assets.*)

Sprite loading was a bare png_load — one file per 16x16 sprite, no way to load
one sheet and address a cell by grid coords or name. Adds a Sprite.*/Assets.*
runtime (atlas.ludic) over the variable-size image loader, so a cell is a
sub-rect of the kept image and is NOT restricted to the 16x16 sprite table:
Sprite.sheet(path,cw,ch), Sprite.cell(sheet,col,row),
Sprite.cell_span(sheet,col,row,cols,rows) (a sprite may span >1 cell),
Sprite.define/named (name + lookup), Sprite.draw/draw_scaled (through
camera/zoom/clip like Screen.sprite), Sprite.width/height, and
Assets.image/load/get. Spliced on demand (Sprite.sheet/… or Assets.*), so a
program using neither is byte-identical.

Example examples/library/atlas.ludic (verified against a real 12x11 Kenney
sheet, incl. a 2x3 multi-cell span and named lookup). 14 docs pages. Full
suite 114/0, goldens byte-identical, fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

changes/spritesheet-atlas.md

@@ -0,0 +1,3 @@
+bump: minor
+type: feat
+Namespaced spritesheet / atlas API (#81). Sprite loading was a bare `png_load("floor0.png")` — one file per 16x16 sprite, with no way to load one sheet and address a cell by grid coords or name. Adds a `Sprite.*` / `Assets.*` runtime (over the variable-size image loader, so a cell is a sub-rect of the kept image and is **not** restricted to the 16x16 sprite table): `Sprite.sheet(path, cellw, cellh)` -> handle, `Sprite.cell(sheet, col, row)` and `Sprite.cell_span(sheet, col, row, cols, rows)` -> id (a sprite may span more than one cell — a tall character, a wide object), `Sprite.define(name, …)` / `Sprite.named(name)` to name and look up a cell, `Sprite.draw` / `Sprite.draw_scaled` (through the camera / zoom / clip, like `Screen.sprite`), `Sprite.width` / `height`, and `Assets.image(path)` / `Assets.load` / `Assets.get(name)`. Spliced on demand; a program that uses neither compiles byte-identically. Example: `examples/library/atlas.ludic`.