perf(cms): batch layout bootstrap, sparse fieldsets, depth caps

Adopt RustyCMS' new resolve semantics (_depth, _fields,
POST /_batch) where they pay off:

- +layout.server.ts: 4 sequential GETs (navHeader, navSocial,
  footer, page_config) collapsed into one batchFetch() call.
  Legacy "default" page_config slug stays as fallback Single-
  Batch only when the canonical slug is missing.
- New getPageStubs() / getPostStubs() use _fields= so the
  Slug→Label/Slug→URL maps don't pull body/rows. Typically
  10-50× smaller payload, eligible for the same TTL cache.
- loadPostsList() now requests posts with _depth=2 and a
  sparse fieldset that keeps card-level fields plus
  postImage.* and postTag.{name,icon,color,_slug}. Body
  Markdown stays out of the listing entirely.
- Page + post detail routes set _depth=3 as a cycle/blast-radius
  cap (page → block → image → img reaches in three levels).
- cms.ts: ContentGetOptions gains depth/fields, getPageBySlug /
  getPostBySlug / getPosts thread them through (cache key
  extended). New batchFetch() / batchData<T>() helpers.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/post/[slug]/+page.server.ts

@@ -35,6 +35,8 @@ export const load: PageServerLoad = async ({ params, locals, url, setHeaders })
   const post = await getPostBySlug(slug, {
     locale: 'de',
     resolve: POST_RESOLVE,
+    // Hard-cap: post → block → image → img reicht in 3 Levels.
+    depth: 3,
     preview,
   });