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/blog.sx

@@ -330,6 +330,18 @@
       (host/blog--uniq
         (reduce (fn (acc t) (concat acc (host/blog-in t "is-a"))) (list) subtypes)))))
 
+;; All type-posts: the subtype-of hierarchy rooted at "type" (type + its transitive
+;; subtypes). This is "the types you've DEFINED" — distinct from host/blog-instances-of
+;; "type" (which is the is-a INSTANCES of the type, i.e. typed content, not the type
+;; definitions; the definitions are linked by subtype-of, the same set instances-of
+;; computes internally as `subtypes`). Used by the metamodel overview + editor.
+(define host/blog-type-defs
+  (fn ()
+    (host/blog--uniq
+      (concat (list "type")
+        (host/blog--edge-slugs
+          (relations/ancestors (host/blog--node "type") (string->symbol "subtype-of")))))))
+
 ;; ── Slice 4: type ALGEBRA — intersection (∧) and union (∨) types ─────
 ;; An algebraic type is a post with operand edges: a `conj` edge per intersection
 ;; member, a `disj` edge per union member. Its EXTENT is its operands' extents combined
@@ -960,7 +972,8 @@
                   (unquote listing)
                   (p (a :href "/new" "+ New post"))
                   (p :style "margin-top:2em;font-size:0.9em;opacity:0.8"
-                    (unquote auth-foot)))))))))))
+                    (a :href "/meta" "metamodel") " · " (a :href "/tags" "tags")
+                    " · " (unquote auth-foot)))))))))))
 
 (define host/blog-index (fn (req) (host/ok (host/blog-list))))
 
@@ -988,6 +1001,62 @@
                 (p :style "margin-top:2em;font-size:0.9em;opacity:0.8"
                   (a :href "/" "all posts") " · " (unquote auth-foot))))))))))
 
+;; ── metamodel overview (GET /meta) ─────────────────────────────────
+;; The "see the system you've defined" page: every type-post (with its schema's
+;; required blocks) and every relation-post (with its signature). Types + relations
+;; are themselves posts, so each row links to the post that defines it. Pure read;
+;; durable reads pre-fetched into let bindings BEFORE the quasiquote (a perform during
+;; tree-build raises VmSuspended), and relations come from the boot-populated
+;; host/blog-rel-kinds VALUE (no perform). The surface the metamodel editor hangs off.
+(define host/blog--schema-summary
+  (fn (schema)
+    (if (and schema (> (len schema) 0))
+      (join ", " (map (fn (rule) (get rule :block)) schema))
+      "—")))
+(define host/blog-meta-index
+  (fn (req)
+    (let ((type-recs
+            (map (fn (s) {:slug s
+                          :title (get (host/blog-get s) :title)
+                          :schema (host/blog-schema-of s)})
+              (sort (host/blog-type-defs))))
+          (rel-specs host/blog-rel-kinds)
+          (auth-foot (host/auth-footer req)))
+      (let ((type-rows
+              (map (fn (p)
+                     (quasiquote
+                       (tr (td (a :href (unquote (str "/" (get p :slug) "/"))
+                                 (unquote (get p :title))))
+                           (td (unquote (host/blog--schema-summary (get p :schema)))))))
+                type-recs))
+            (rel-rows
+              (map (fn (spec)
+                     (quasiquote
+                       (tr (td (unquote (get spec :kind)))
+                           (td (unquote (or (get spec :label) "")))
+                           (td (unquote (if (get spec :symmetric) "symmetric" "directed")))
+                           (td (unquote (or (get spec :inverse-label) "—"))))))
+                rel-specs)))
+        (host/blog--resp req 200
+          (host/blog--page req "Metamodel"
+            (quasiquote
+              (div
+                (h1 "Metamodel")
+                (p "The types and relations that define this system. Each is itself a post — click through to its definition.")
+                (h2 (unquote (str "Types (" (len type-recs) ")")))
+                (unquote (if (> (len type-recs) 0)
+                           (cons (quote table)
+                             (cons (quote (tr (th "Type") (th "Required blocks"))) type-rows))
+                           (quote (p "No types yet."))))
+                (h2 (unquote (str "Relations (" (len rel-specs) ")")))
+                (unquote (if (> (len rel-specs) 0)
+                           (cons (quote table)
+                             (cons (quote (tr (th "Relation") (th "Label") (th "Kind") (th "Inverse"))) rel-rows))
+                           (quote (p "No relations yet."))))
+                (p :style "margin-top:2em;font-size:0.9em;opacity:0.8"
+                  (a :href "/" "all posts") " · " (a :href "/tags" "tags")
+                  " · " (unquote auth-foot))))))))))
+
 ;; GET /<slug>/source — the raw sx_content as text/plain. Posts ARE SX source, so
 ;; this just hands back the stored markup (public; a published post's source is
 ;; not secret). 404 if the post is absent.
@@ -1206,6 +1275,7 @@
     (dream-get "/posts" host/blog-index)
     (dream-get "/new" host/blog-new-form)
     (dream-get "/tags" host/blog-tags-index)
+    (dream-get "/meta" host/blog-meta-index)
     (dream-get "/:slug/source" host/blog-source)
     (dream-get "/:slug/relate-options" host/blog-relate-options)
     (dream-get "/:slug" host/blog-post)))

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)"