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/model/Project.ts

@@ -1,3 +1,4 @@
+import { schemaForAsset } from '../properties/PropertySchema';
 import { type Asset, type ContainerAsset, isSceneAsset, type SceneAsset } from './Asset';
 import type { SceneObject } from './SceneObject';
 
@@ -63,3 +64,40 @@ export function objectsWhere(
 export function findUsages(project: Project, assetId: string): SceneObjectRef[] {
   return objectsWhere(project, (o) => o.assetId === assetId);
 }
+
+/** One place where an `asset`-kind property points at an asset. */
+export type PropertyUsage =
+  | { on: 'asset'; asset: Asset; key: string }
+  | { on: 'object'; holder: ObjectHolder; object: SceneObject; key: string };
+
+/**
+ * Every property that references `assetId`, on assets and on the overrides of placed objects.
+ *
+ * This is the other half of "where is this asset used": `findUsages` finds assets that were
+ * *placed*, which is meaningless for a texture – a texture is never placed, it is worn by
+ * sprites through their `texture` property. Deleting one therefore has to count both, or it
+ * reports "0 uses" for a picture that ten sprites are showing.
+ *
+ * A reference is deliberately left dangling rather than cleared when its asset goes: a
+ * dangling one resolves to "no picture" everywhere it is read, and it comes back by itself
+ * when the delete is undone.
+ */
+export function findPropertyUsages(project: Project, assetId: string): PropertyUsage[] {
+  if (!assetId) return [];
+  const result: PropertyUsage[] = [];
+  const assetKeys = new Map<string, string[]>();
+  for (const asset of project.assets) {
+    const keys = schemaForAsset(asset)
+      .filter((def) => def.kind === 'asset')
+      .map((def) => def.key);
+    assetKeys.set(asset.id, keys);
+    for (const key of keys) if (asset.props[key] === assetId) result.push({ on: 'asset', asset, key });
+  }
+  for (const holder of objectHolders(project)) {
+    for (const object of holder.objects) {
+      for (const key of assetKeys.get(object.assetId) ?? [])
+        if (object.overrides[key] === assetId) result.push({ on: 'object', holder, object, key });
+    }
+  }
+  return result;
+}