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/assets/_section.md

@@ -0,0 +1,7 @@
+---
+id: assets
+title: Assets
+order: 34
+---
+
+Loading and lookup for whole-file assets, paired with the <a href="sprite-sheet"><code>Sprite.*</code></a> atlas API. <a href="assets-image"><code>Assets.image</code></a> (and its alias <a href="assets-load"><code>Assets.load</code></a>) loads a whole image file as one sprite; <a href="assets-get"><code>Assets.get</code></a> looks a named sprite up.

docs/language/assets/assets-get.md

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+---
+id: assets-get
+name: Assets.get
+category: assets
+kind: namespace-method
+tokens: Assets.get
+sig: Assets.get(name) -> int
+tip: Look up a named sprite's id (alias of Sprite.named).
+order: 3
+ns: Assets
+member: get
+---
+
+Alias of <a href="sprite-named"><code>Sprite.named</code></a>: returns the sprite id registered under <code>name</code>, or <code>-1</code>.

docs/language/assets/assets-image.md

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+---
+id: assets-image
+name: Assets.image
+category: assets
+kind: namespace-method
+tokens: Assets.image
+sig: Assets.image(path) -> int
+tip: Load a whole image file as one sprite; returns its id.
+order: 1
+ns: Assets
+member: image
+---
+
+Loads the image at <code>path</code> as a single atlas sprite the size of the image, returning its id — the namespaced replacement for a bare <code>png_load</code>. Draw it with <a href="sprite-draw"><code>Sprite.draw</code></a>.

docs/language/assets/assets-load.md

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+---
+id: assets-load
+name: Assets.load
+category: assets
+kind: namespace-method
+tokens: Assets.load
+sig: Assets.load(path) -> int
+tip: Alias of Assets.image.
+order: 2
+ns: Assets
+member: load
+---
+
+Alias of <a href="assets-image"><code>Assets.image</code></a>: loads a whole image file as one sprite and returns its id.