Let the agent draw pixel art and vector art (#429)

Stage 5c of the MCP work. The agent could place sprites and edit
graphs, but it could not draw: pixel art, palettes and vector art had
no commands in its catalog. Two new areas bring 29 commands, so the
catalog now holds 144.

- pixelart (13): paint, layers (add, copy, remove, change, move,
  merge down), frames (add, also as a copy, remove, move), canvas size
  with an anchor, palette colors set and removed.
- paint_pixels edits one layer on one frame. Its steps run in order:
  rows (a small picture as characters, a space keeps the pixel),
  pixels, line, rect, ellipse, fill, clear. An index the palette does
  not have, a pixel off the canvas and an unknown character are
  refused.
- vectorart (16): shapes from rings or as rect and ellipse, outline,
  style (fill, opacity, gradient or image fill, stroke), move, scale,
  rotate and flip, copy, remove, combine, rename, order, move into
  groups, group, layers.
- get_asset gives the palette of pixel art as colors. Up to 16384
  pixels over all layers and frames each layer comes as rows, above
  that only the number of painted pixels. Palette and vector colors
  come as #rrggbb.
- Merge down, canvas size and removing a color are now builders in
  pixelActions.ts. The panel and the agent share the same code.
- Color and point helpers moved into catalog/shared.ts, and graphs.ts
  uses them too.

Found while testing:

- The ellipse tests only checked the four extreme points, which stay
  right even with wrong handle lengths. The test now checks that every
  point of the drawn curve lies on the ellipse.

Tests:

- agent-drawing (new, 9) and the other agent, pixel and relay tests:
  133/133.
- 7 mutations, all red in the matching test.
- e2e 71 on melektaus-atx: 24/24. New check: the agent's pixel art
  opens in its tab, reads back as rows, and its cel is on the server
  after saving (status 200).
- Live in a test project: an 8x8 sprite and a copied second frame
  read back as painted, a rotated square and a radial gradient disc
  with their corners where computed.
- Gate on melektaus-atx: CHECK EXIT 0, 322 files, 4978 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/agent/tools.json

@@ -44,7 +44,7 @@
   {
     "name": "get_asset",
     "title": "Read an asset",
-    "description": "One asset as stored: its property values, the rows of its properties (key, kind, default), the objects of a scene or container with their names and positions (the content of a placed container too, marked fromContainer), what the rows of an animation may aim at (targets), the cells of a small tilemap. A graph is only counted here; read it with get_graph.",
+    "description": "One asset as stored: its property values, the rows of its properties (key, kind, default), the objects of a scene or container with their names and positions (the content of a placed container too, marked fromContainer), what the rows of an animation may aim at (targets), the cells of a small tilemap, the pixels of small pixel art as rows of palette indices (colors as \"#rrggbb\"). A graph is only counted here; read it with get_graph.",
     "inputSchema": {
       "type": "object",
       "properties": { "asset": { "type": "string", "description": "Id of the asset." } },