fix: bundle mosaics broken since find date-bucketing — index find/<yyyy>/<mm>/<slug> paths, add avif

The generator still resolved heroes at the old flat src/content/find/<slug>/
path, so every mosaic was skipped as 'no heroes' and src/generated/
bundle-mosaics sat empty. Home-page bundle tiles have been mosaic-less since
the May migration.

scripts/build-bundle-mosaics.mjs

@@ -23,7 +23,7 @@ const TILE = 598;
 const GAP = 4;
 const FULL = TILE * 2 + GAP; // 1200
 const BG = { r: 224, g: 219, b: 210, alpha: 1 }; // matches light --rule
-const HERO_EXTS = ['jpg', 'jpeg', 'png', 'webp', 'gif'];
+const HERO_EXTS = ['jpg', 'jpeg', 'png', 'webp', 'gif', 'avif'];
 
 function parseItems(frontmatter) {
   const lines = frontmatter.split('\n');
@@ -46,9 +46,28 @@ async function readFrontmatter(path) {
   return m ? m[1] : '';
 }
 
+// Finds live at find/<year>/<month>/<slug>/ (slugs globally unique).
+// Index them once so hero lookup stays O(1) per slug.
+let findDirIndex = null;
+async function buildFindDirIndex() {
+  const index = new Map();
+  const years = (await readdir(FIND_DIR, { withFileTypes: true })).filter((d) => d.isDirectory());
+  for (const year of years) {
+    const yearDir = join(FIND_DIR, year.name);
+    const months = (await readdir(yearDir, { withFileTypes: true })).filter((d) => d.isDirectory());
+    for (const month of months) {
+      const monthDir = join(yearDir, month.name);
+      const slugs = (await readdir(monthDir, { withFileTypes: true })).filter((d) => d.isDirectory());
+      for (const slug of slugs) index.set(slug.name, join(monthDir, slug.name));
+    }
+  }
+  return index;
+}
+
 async function findHeroPath(slug) {
-  const dir = join(FIND_DIR, slug);
-  if (!existsSync(dir)) return null;
+  if (!findDirIndex) findDirIndex = await buildFindDirIndex();
+  const dir = findDirIndex.get(slug);
+  if (!dir) return null;
   const entries = await readdir(dir);
   for (const ext of HERO_EXTS) {
     const file = entries.find((e) => e.toLowerCase() === `hero.${ext}`);