host: metamodel overview page (GET /meta) — the first editor surface

The 'see the system you've defined' page: every type-post (with its schema's required
blocks) and every relation-post (with its signature), each linking to the post that
defines it. The surface the metamodel editor hangs off (North Star UI surface 1 of 3).

- host/blog-type-defs: the type DEFINITIONS = the subtype-of hierarchy rooted at 'type'
  (type + transitive subtypes). NOT host/blog-instances-of 'type' (that's the is-a
  INSTANCES — typed content, not the definitions, which are linked by subtype-of).
- host/blog-meta-index (GET /meta, mounted before /:slug): pure read, all durable reads
  pre-fetched into let bindings before the quasiquote (perform-in-tree = VmSuspend);
  relations from the boot-populated host/blog-rel-kinds VALUE. Types + relations tables.
- Home footer links to /meta + /tags.

Verified live (ephemeral): Types (3: Type/Tag/Article, Article shows required block h1),
Relations (4: related symmetric, is-a/subtype-of/tagged directed). Blog suite 122/122.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/tests/blog.sx

@@ -588,6 +588,20 @@
     (list (host/blog-type-valid? "art1" "(p \"no heading\")")
           (host/blog-type-valid? "art1" "(article (h1 \"H\") (p \"x\"))")))
   (list false true))
+
+;; -- metamodel overview (GET /meta) --
+(host-bl-test "type-defs = the subtype hierarchy (type defs), not is-a instances"
+  (let ((defs (host/blog-type-defs)))
+    (list (contains? defs "type") (contains? defs "article") (contains? defs "art1")))
+  (list true true false))
+(host-bl-test "/meta is 200 (not shadowed by /:slug)"
+  (dream-status (host-bl-app (host-bl-req "/meta"))) 200)
+(host-bl-test "/meta lists type definitions + relations + the article's required block"
+  (let ((body (dream-resp-body (host-bl-app (host-bl-req "/meta")))))
+    (list (contains? body "Metamodel") (contains? body "Article")
+          (contains? body "h1") (contains? body "related")
+          (contains? body "symmetric")))
+  (list true true true true true))
 (host-bl-test "a post with no schema'd type is vacuously valid"
   (host/blog-type-valid? "ppost" "(p \"anything\")") true)
 (host-bl-test "edit-submit rejects content violating the type schema (not saved)"