site: rename "finds" superpost collection to "bundles"

Disambiguates the one-letter `find` (item) vs `finds` (roundup) split that
kept causing confusion. Audience-facing now: `/bundles/<slug>/`, "Bundle"
labels in home/footer/search.

Internals: `getCollection('finds')` → `('bundles')`, `Entry.type 'finds'`
→ `'bundle'`, `OgFallback 'finds'` → `'bundles'`, `findListMosaic` →
`bundleMosaic`, `FindsPost` → `Bundle`, `findsReferencing` →
`bundlesReferencing`. Build script + generated mosaic dir follow
(`scripts/build-bundle-mosaics.mjs`, `src/generated/bundle-mosaics/`).

Skills `build-finds` → `build-bundle`, `cluster-finds` →
`cluster-bundles`. The `find` collection (individual items) is unchanged.

No URL redirects: low external-link surface area.

package.json

@@ -4,7 +4,7 @@
   "version": "0.1.0",
   "private": true,
   "scripts": {
-    "build:mosaics": "node scripts/build-list-mosaics.mjs",
+    "build:mosaics": "node scripts/build-bundle-mosaics.mjs",
     "predev": "npm run build:mosaics",
     "prestart": "npm run build:mosaics",
     "prebuild": "npm run build:mosaics",