site: list mosaic tiles for /finds/ + home grid

Adds a build-time mosaic generator (scripts/build-list-mosaics.mjs, sharp)
that composites a single tile image per superpost from its referenced
finds. Wired via predev/prestart/prebuild hooks; output lives in
src/generated/ (gitignored). The /finds/ index and home grid both render
the mosaic when present and fall back to text-only tiles otherwise.

src/lib/find-list-mosaic.ts

@@ -0,0 +1,16 @@
+import type { ImageMetadata } from 'astro';
+
+const modules = import.meta.glob<{ default: ImageMetadata }>(
+  '../generated/list-mosaics/*.jpg',
+  { eager: true }
+);
+
+const mosaics = new Map<string, ImageMetadata>();
+for (const [path, mod] of Object.entries(modules)) {
+  const match = path.match(/list-mosaics\/([^/]+)\.jpg$/);
+  if (match) mosaics.set(match[1], mod.default);
+}
+
+export function findListMosaic(slug: string): ImageMetadata | undefined {
+  return mosaics.get(slug);
+}