Add textures for sprites

Images are uploaded to a content-addressed blob endpoint and stored as
data/textures/<sha256>.<ext>; the project only keeps the file name. The
sprite points at a texture asset through an ordinary property of the new
kind `asset`, so instance overrides, "Apply to container", "Reset to
asset", undo and `Set property` work without any extra code. The new kind
also replaces the ad-hoc asset picker the graph nodes faked as an enum.

Import goes through + -> Texture, drag & drop onto the canvas or Ctrl+V.
Images wider than 2048px are downscaled client-side and re-encoded to
WebP if needed, because nginx caps uploads at 1MB; an image that already
fits is uploaded byte for byte so pixel art is never resampled.

The sprite colour default changes from red to white, since colour tints
the texture multiplicatively and a tint of white shows an image as it is.

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

src/core/store/TextureStore.ts

@@ -0,0 +1,31 @@
+/** What the server made of an uploaded image; the payload a texture asset is built from. */
+export interface UploadedTexture {
+  /** Content-addressed name in `data/textures/`: `{sha256}.{png|jpg|webp|gif}` (see `isTextureFile`). */
+  file: string;
+  width: number;
+  height: number;
+  bytes: number;
+}
+
+/** One stored image as the server lists it. */
+export interface StoredTexture {
+  file: string;
+  bytes: number;
+  updatedAt: string;
+}
+
+/**
+ * Upload boundary for images, the counterpart of `ProjectStore`. The editor only talks to this
+ * interface. An upload is raw bytes rather than a multipart form: name, size and format are
+ * derived from the image itself on the server, so there is nothing else to send.
+ */
+export interface TextureStore {
+  /**
+   * Stores an image and returns its file name and pixel size. `type` only fills the
+   * Content-Type header – the server trusts the bytes, not the label – and uploading the same
+   * picture twice yields the same file, because the name is the hash of its content.
+   */
+  upload(bytes: Blob | ArrayBuffer, type: string): Promise<UploadedTexture>;
+  list(): Promise<StoredTexture[]>;
+  delete(file: string): Promise<void>;
+}