host: typed relations — Phase 1, generalize edges to carry a kind

Plan: plans/typed-posts-and-relations.md. "Typing is just relating to a type",
types are posts. Phase 1 lifts the hard-coded kind:"related" into a parameter,
driven by one registry — the spine the later phases (type resolution, tags,
picker) build on. Zero user-visible change.

- host/blog-rel-kinds registry: {kind,label,symmetric,candidates[,inverse-label]}
  for related (symmetric) / is-a / tagged (directed). One place knows each kind's
  direction, label, and candidate set.
- host/blog-relate!/unrelate! take a kind; symmetric kinds write both directions,
  directed kinds write one. host/blog-out/in read children/parents per kind;
  host/blog-related = out(slug,"related") (back-compat).
- relate/unrelate routes carry a `kind` form field (default "related"), validated
  against the registry. delete drops edges across ALL kinds + both directions.

6 tests: symmetric reads both sides, directed writes one (inverse via host/blog-in),
unrelate is kind-scoped, unknown kind rejected, default kind = related. 244/244;
Playwright picker 4/4 (related path unchanged).

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

lib/host/tests/blog.sx

@@ -267,10 +267,10 @@
   (list true false))
 (host-bl-test "relate-options excludes already-related candidates"
   (begin
-    (host/blog-relate! "alpha-post" "beta-post")
+    (host/blog-relate! "alpha-post" "beta-post" "related")
     (contains? (dream-resp-body (host-bl-app (host-bl-req "/alpha-post/relate-options"))) "Beta Post"))
   false)
-(host/blog-unrelate! "alpha-post" "beta-post")
+(host/blog-unrelate! "alpha-post" "beta-post" "related")
 (host-bl-test "relate-picker.js served as javascript"
   (dream-resp-header (host-bl-app (host-bl-req "/relate-picker.js")) "content-type")
   "application/javascript; charset=utf-8")
@@ -281,6 +281,40 @@
 (host-bl-test "related block: empty when anonymous + no relations"
   (= (host/blog--related-block "gamma-post" false) "") true)
 
+;; -- Phase 1: relations carry a kind --
+(host-bl-test "symmetric kind (related) reads from both sides"
+  (begin
+    (host/blog-relate! "alpha-post" "gamma-post" "related")
+    (list (contains? (host/blog-out "alpha-post" "related") "gamma-post")
+          (contains? (host/blog-out "gamma-post" "related") "alpha-post")))
+  (list true true))
+(host-bl-test "directed kind (tagged) writes one direction; inverse via host/blog-in"
+  (begin
+    (host/blog-relate! "alpha-post" "beta-post" "tagged")
+    (list (contains? (host/blog-out "alpha-post" "tagged") "beta-post")
+          (contains? (host/blog-out "beta-post" "tagged") "alpha-post")
+          (contains? (host/blog-in "beta-post" "tagged") "alpha-post")))
+  (list true false true))
+(host-bl-test "unrelate is kind-scoped (related edge survives a tagged unrelate)"
+  (begin
+    (host/blog-unrelate! "alpha-post" "beta-post" "tagged")
+    (list (contains? (host/blog-out "alpha-post" "tagged") "beta-post")
+          (contains? (host/blog-out "alpha-post" "related") "gamma-post")))
+  (list false true))
+(host/blog-unrelate! "alpha-post" "gamma-post" "related")
+(host-bl-test "relate-submit rejects an unknown kind (no-op)"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/alpha-post/relate" "Bearer good"
+      "application/x-www-form-urlencoded" "other=beta-post&kind=bogus"))
+    (contains? (host/blog-out "alpha-post" "bogus") "beta-post"))
+  false)
+(host-bl-test "default kind is related (no kind field)"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/alpha-post/relate" "Bearer good"
+      "application/x-www-form-urlencoded" "other=beta-post"))
+    (contains? (host/blog-out "alpha-post" "related") "beta-post"))
+  true)
+
 ;; -- 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))