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-cell.md

@@ -0,0 +1,14 @@
+---
+id: sprite-cell
+name: Sprite.cell
+category: sprite
+kind: namespace-method
+tokens: Sprite.cell
+sig: Sprite.cell(sheet, col, row) -> int
+tip: One cell of a sheet, by grid coords; returns a sprite id.
+order: 2
+ns: Sprite
+member: cell
+---
+
+Returns a sprite id for the cell at column <code>col</code>, row <code>row</code> of <code>sheet</code> (the sub-rect <code>(col*cellw, row*cellh, cellw, cellh)</code>). Draw it with <a href="sprite-draw"><code>Sprite.draw</code></a>.