Pixel art can be drawn in its own tab with layers and frames (#130)

Textures are uploaded files named by their content, so they cannot
be edited in place. Pixel art is therefore its own asset type, like
vector art. Its pixels live in the project and the tab rasters them
into a texture that sprites, tiles and animations can use.

- New asset types Pixel art and Palette. The first pixel art of a
  project gets a palette with the 16 PICO-8 colours in the same undo
  step. Every pixel is a palette index, 0 is transparent.
- Each layer on each frame is its own file data/pixels/<sha256>.px.
  The project only holds the names, so undo swaps names. The browser
  computes the hash and uploads new cels before the project is saved.
- New route POST ?r=pixels in api/lib/PixelStore.php. It answers 201
  and 422 for broken data. data/pixels/ is git-ignored.
- Tools: pencil, eraser, fill (contiguous or all pixels of a colour),
  eyedropper, line, rectangle and ellipse (outline or filled), brush
  size 1 to 8.
- Layers can be added, deleted, renamed, reordered, hidden, faded and
  merged down. Frames can be added, duplicated, deleted and reordered,
  with onion skin and a small playing preview.
- Zoom 1x to 32x in whole steps, grid from 8x, a 3x3 tile preview and
  Resize... with a 3x3 anchor.
- All frames sit side by side in one image. Each frame is a region
  like a cut texture frame, so texture rows offer "Asset / Frame 2"
  and the context menu has Create animation from frames.

Decisions:
- Merging down drops the opacity of the upper layer. Indexed colours
  know no blends.
- Only the last palette colour can be removed. Otherwise the indices
  of every image using the palette would shift.
- Undo stores the whole layer per stroke instead of pixel diffs.
- Alt+click picks a colour. It is a mouse gesture, not a shortcut.

Follow-up issues: #197 import from texture, #198 selection and
flipping, #199 PNG export, #200 more tools.

Tests:
- ./check.sh: 136 files, 2049 tests (before 135 / 2011). 34 new in
  tests/core/pixel-art.test.ts, 4 from the icon tables.
- e2e module 48 fresh: ALL PASSED, 29 checks, console empty.
- Counter-probes bite: five on the unit tests, upload order (2 tests
  fail) and cels not uploaded in the browser (1 check fails).
- Check total 1259 in README is computed (1232 measured in CI + 27).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/lib/scaffold.mjs

@@ -26,6 +26,7 @@ export const LANES = {
   queries: -4300,
   symbols: -4700,
   vectorArt: -5100,
+  pixelArt: -5400,
 };
 
 /**
@@ -115,6 +116,7 @@ export const TAB_LABELS = {
   shader: 'Shader',
   texture: 'Texture',
   vectorart: 'Vector art',
+  pixelart: 'Pixel art',
   animation: 'Animation',
   tile: 'Tile',
   container: 'Container',
@@ -163,6 +165,7 @@ const DOCUMENT_KINDS = [
   'particles',
   'texture',
   'vectorart',
+  'pixelart',
   'animation',
   'tile',
   'container',