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>

docs/language/sprite/sprite-sheet.md

@@ -0,0 +1,14 @@
+---
+id: sprite-sheet
+name: Sprite.sheet
+category: sprite
+kind: namespace-method
+tokens: Sprite.sheet
+sig: Sprite.sheet(path, cellw, cellh) -> int
+tip: Load a spritesheet and remember its cell grid; returns a sheet handle.
+order: 1
+ns: Sprite
+member: sheet
+---
+
+Loads the PNG at <code>path</code> and records that its cells are <code>cellw x cellh</code> px, returning a sheet <em>handle</em>. Address its cells with <a href="sprite-cell"><code>Sprite.cell</code></a> / <a href="sprite-cell_span"><code>Sprite.cell_span</code></a>. The whole image is kept (variable size), so cells are not restricted to the 16x16 sprite table.