Read the pixel size of a texture in a graph (#262)

A graph had no way to know how big a picture is. An icon sprite with a
fixed size therefore stretched every icon of another size, an 8x8
pixel-art icon in a 16x16 slot was drawn twice as big.

- New data node Texture size. It takes a texture, a frame or a pixel
  art and hands out width and height in pixels, 0 x 0 for none.
  Wired into a sprite's width and height, the picture shows pixel for
  pixel.
- GraphLookup gets textureSize. The project answers it with the same
  pictureSize rule a sprite uses to size itself.

Tests:

- ./check.sh: 2749 tests, CHECK EXIT 0.
- 2 catalog cases (a picture, no picture) and one test against a real
  project: pixel-art sheet, single frame, missing texture.
- No e2e check and no counter-check for this node.

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

src/core/graph/lookup.ts

@@ -46,6 +46,8 @@ export interface GraphLookup {
   curve(assetId: string): readonly VectorPoint[] | null;
   /** A Struct asset as a run knows it: a name and field names, nothing else travels (#254). */
   structType(assetId: string): StructShape | null;
+  /** A texture's, frame's or pixel-art frame's own size in pixels; null for no picture. */
+  textureSize(pictureId: string): { w: number; h: number } | null;
 }
 
 export interface StructShape {
@@ -58,4 +60,5 @@ export const NO_LOOKUP: GraphLookup = {
   machine: () => null,
   curve: () => null,
   structType: () => null,
+  textureSize: () => null,
 };