host: typed relations — Phase 4 cleanup, registry-driven render + /tags

Replace the hard-coded related/tagged blocks with iteration over the registry,
so adding a kind renders automatically — no handler edit.

- host/blog--relation-blocks: iterates host/blog-rel-kinds; each kind contributes
  its outgoing block (label) and, if it has an inverse, its incoming block
  (inverse-label, e.g. tagged -> "Tagged with this", is-a -> "Instances"). Empty
  blocks dropped; one kv-keys read up front, relation lookups in-memory.
  host/blog--relations-or-hint adds the logged-in "add some" hint when empty.
- host/blog--relation-editors: one editor per registry kind on the edit page
  (Related / Types / Subtype of / Tags), replacing the hard-coded two.
- GET /tags: index of every tag (a post that is-a tag), each linking its own page.
- dropped host/blog--related-block / --kind-block / --tagged-with-block (folded
  into host/blog--edges-block + the registry iteration).
- GOTCHA (4th time): host/blog-tags-index called host/blog-get INSIDE the item
  quasiquote -> VmSuspended/500 live (conformance in-memory store can't see it);
  pre-fetch records before the quasiquote.

5 tests (relations-section hint, registry render of Related+Tags, inverse block
for a tag, /tags lists + 200). 265/265; Playwright 4/4. Verified live: /tags,
post pages show registry blocks, tag page shows Types + Tagged-with-this, edit
page has a picker per kind.

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

lib/host/tests/blog.sx

@@ -276,10 +276,11 @@
   "application/javascript; charset=utf-8")
 (host-bl-test "relate-picker.js carries the fetch glue"
   (contains? (dream-resp-body (host-bl-app (host-bl-req "/relate-picker.js"))) "relate-options") true)
-(host-bl-test "related block: hint when logged-in + no relations"
-  (contains? (str (host/blog--related-block "gamma-post" true)) "add some") true)
-(host-bl-test "related block: empty when anonymous + no relations"
-  (= (host/blog--related-block "gamma-post" false) "") true)
+(host/blog-put! "hint-post" "Hint Post" "(p \"h\")" "published")
+(host-bl-test "relations section: hint when logged-in + no relations"
+  (contains? (str (host/blog--relations-or-hint "hint-post" true)) "add some") true)
+(host-bl-test "relations section: empty when anonymous + no relations"
+  (= (host/blog--relations-or-hint "hint-post" false) "") true)
 
 ;; -- Phase 1: relations carry a kind --
 (host-bl-test "symmetric kind (related) reads from both sides"
@@ -395,6 +396,21 @@
     (host/blog-is-tag? "pdoc"))
   true)
 
+;; -- Phase 4: registry-driven render + /tags index --
+(host-bl-test "relation-blocks renders Related + Tags from the registry"
+  (begin
+    (host/blog-relate! "hint-post" "ppost" "related")
+    (host/blog-relate! "hint-post" "ocaml" "tagged")
+    (let ((body (str (host/blog--relation-blocks "hint-post"))))
+      (list (contains? body "Related posts") (contains? body "Tags"))))
+  (list true true))
+(host-bl-test "relation-blocks shows an inverse block (Tagged with this) for a tag"
+  (contains? (str (host/blog--relation-blocks "ocaml")) "Tagged with this") true)
+(host-bl-test "/tags lists the tag posts"
+  (contains? (dream-resp-body (host-bl-app (host-bl-req "/tags"))) "OCaml") true)
+(host-bl-test "/tags is 200 (not shadowed by /:slug)"
+  (dream-status (host-bl-app (host-bl-req "/tags"))) 200)
+
 ;; -- experimental unguarded create-only route (POST /new, no auth) --
 (define host-bl-oapp (host/make-app (list host/blog-open-create-routes host/blog-routes)))
 (host/blog-use-store! (persist/open))