host: relate posts — "related posts" on blog × relations (blog 61/61, 230)

Compose two already-migrated domains: a post is a relations-graph node
"blog:<slug>", and a "related" link is a symmetric pair of edges
(lib/relations). The post page shows a "Related posts" block; the edit page
gets an editor to add (by slug) and remove relations.

- host/blog-relate!/unrelate!/related: symmetric edges under kind "related";
  related slugs = blog children, existence-filtered against ONE kv-keys read.
- post page: "Related posts" links block; edit page: related editor (remove
  buttons + add-by-slug box).
- POST /:slug/relate, /:slug/unrelate — guarded browser routes (redirect to
  login like the other write routes); relate validates the other post exists.
- delete cleans up a post's related edges (no dangling links).

IO ORDERING (the live 500 that conformance missed): host/blog--related-block/
-editor do durable reads (perform). Performing inside the quasiquote, via
unquote, while the page tree renders raised Sx_vm.VmSuspended under http-listen;
the in-memory conformance store never performs, so it passed. Fix mirrors
host/blog-home: do the reads in the handler's let bindings BEFORE the
quasiquote, and check related-existence against a single host/blog-slugs read
rather than a perform per candidate inside filter.

9 relate tests (guard, symmetry, render, no-op on missing, unrelate both ways,
delete cleanup). Verified live: relate -> Related block both ways; unrelate
clears it; posts without relations and the whole site stay 200.

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

lib/host/tests/blog.sx

@@ -211,6 +211,44 @@
 (host-bl-test "GET /logout -> 303"
   (dream-status (host-bl-app (host-bl-req "/logout"))) 303)
 
+;; -- relate posts (blog × relations) --
+;; my-first-post and another-one both exist in the write-test store at this point.
+(host-bl-test "relate no auth -> redirect to login"
+  (dream-status (host-bl-wapp (host-bl-send "POST" "/my-first-post/relate" nil
+    "application/x-www-form-urlencoded" "other=another-one"))) 303)
+(host-bl-test "relate authed -> 303 back to edit"
+  (dream-resp-header (host-bl-wapp (host-bl-send "POST" "/my-first-post/relate" "Bearer good"
+    "application/x-www-form-urlencoded" "other=another-one")) "location")
+  "/my-first-post/edit")
+(host-bl-test "related is symmetric (a -> b)"
+  (contains? (host/blog-related "my-first-post") "another-one") true)
+(host-bl-test "related is symmetric (b -> a)"
+  (contains? (host/blog-related "another-one") "my-first-post") true)
+(host-bl-test "post page shows a Related posts block"
+  (contains? (dream-resp-body (host-bl-wapp (host-bl-req "/my-first-post/"))) "Related posts") true)
+(host-bl-test "post page links the related post"
+  (contains? (dream-resp-body (host-bl-wapp (host-bl-req "/my-first-post/"))) "/another-one/") true)
+(host-bl-test "relate nonexistent other -> no-op"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/my-first-post/relate" "Bearer good"
+      "application/x-www-form-urlencoded" "other=ghost-post"))
+    (contains? (host/blog-related "my-first-post") "ghost-post"))
+  false)
+(host-bl-test "unrelate -> removes the link both ways"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/my-first-post/unrelate" "Bearer good"
+      "application/x-www-form-urlencoded" "other=another-one"))
+    (list (contains? (host/blog-related "my-first-post") "another-one")
+          (contains? (host/blog-related "another-one") "my-first-post")))
+  (list false false))
+(host-bl-test "delete cleans up related edges"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/my-first-post/relate" "Bearer good"
+      "application/x-www-form-urlencoded" "other=another-one"))
+    (host-bl-wapp (host-bl-send "DELETE" "/posts/another-one" "Bearer good" "" ""))
+    (contains? (host/blog-related "my-first-post") "another-one"))
+  false)
+
 ;; -- 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))