host: relations-as-posts slice 1 — declaration-driven candidate pools

Types declare which relation they anchor (type declares is-a/subtype-of, tag
declares tagged) via a 'declares' edge; the picker's candidate set is the
down-closure of a relation's anchors through is-a ∪ subtype-of. So is-a/subtype-of
now offer the WHOLE type closure — the roots (type/tag/article) AND instances —
fixing the wrinkle where only instances showed and you could never pick 'tag' or
'article' as a type. 'related' has no anchor → every post.

Replaces the hardcoded :candidates "types"/"tags"/"all" with graph queries
(host/blog--reach-down + the declares edges). Design + roadmap (relations as
first-class posts, typed relations, type algebra, constraints) in
plans/relations-as-posts.md.

host conformance 283/283 (+5: is-a pool includes type roots, excludes plain posts,
tagged anchored by tag, related = all, is-a relate-options offers Article).

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

lib/host/blog.sx

@@ -322,27 +322,55 @@
       (host/blog-seed! "article" "Article"
         "(article (h1 \"Article\") (p \"A kind of post that must have a heading. A post that is-a article is checked against this type's schema on save — gradual typing: declaring the type adds the requirement, and the next edit must satisfy it.\"))"
         "published")
-      (host/blog-relate! "article" "type" "subtype-of"))))
+      (host/blog-relate! "article" "type" "subtype-of")
+      ;; relation DECLARATIONS (see plans/relations-as-posts.md). A type-post declares
+      ;; which relation it anchors at its OBJECT end ("you may point at me with R"); the
+      ;; picker's candidate set is the down-closure of a relation's anchors through the
+      ;; type graph, so the candidates for a relation are exactly the posts that inherit
+      ;; its declaration. `type` anchors is-a + subtype-of (you point at a type), `tag`
+      ;; anchors tagged (you point at a tag). `related` has no anchor → every post.
+      (host/blog-relate! "type" "is-a" "declares")
+      (host/blog-relate! "type" "subtype-of" "declares")
+      (host/blog-relate! "tag" "tagged" "declares"))))
 
 ;; ── relate picker (filterable, paginated candidate list) ────────────
 ;; Candidates to relate `slug` to: every post except itself and ones already
 ;; related, narrowed by `q` (case-insensitive substring of title or slug),
 ;; title-sorted. One page is `host/blog--picker-limit` rows from `offset`.
 (define host/blog--picker-limit 20)
-;; The candidate POOL for a kind comes from its registry :candidates: "all" posts,
-;; or the members of a type ("tags" = instances of tag, "types" = instances of
-;; type). Enumerating a type's members is O(#subtypes), not O(#posts).
+;; Down-closure: every post reachable from `roots` by walking INVERSE is-a ∪
+;; subtype-of edges (i.e. instances and subtypes, transitively), roots included.
+;; This is "everything that is, transitively, an instance-or-subtype of a root".
+;; BFS over direct edges (host/blog-in); `seen` makes it cycle-safe and terminating.
+(define host/blog--reach-down
+  (fn (roots)
+    (let loop ((frontier roots) (seen (list)))
+      (if (empty? frontier)
+        seen
+        (let ((t (first frontier)))
+          (if (contains? seen t)
+            (loop (rest frontier) seen)
+            (loop
+              (concat (rest frontier)
+                (concat (host/blog-in t "is-a") (host/blog-in t "subtype-of")))
+              (concat seen (list t)))))))))
+
+;; The candidate POOL for relating under `kind` is DECLARATION-driven (see
+;; plans/relations-as-posts.md): the down-closure of the posts that DECLARE `kind`
+;; at their object end. So is-a/subtype-of (anchored by `type`) offer the whole type
+;; closure — roots AND instances — and `tagged` (anchored by `tag`) offers the tags.
+;; A relation with no declaration (e.g. `related`) offers every post.
 (define host/blog--candidate-pool
-  (fn (candidates)
-    (cond
-      ((= candidates "tags") (host/blog-instances-of "tag"))
-      ((= candidates "types") (host/blog-instances-of "type"))
-      (else (host/blog-slugs)))))
+  (fn (kind)
+    (let ((anchors (host/blog-in kind "declares")))
+      (if (empty? anchors)
+        (host/blog-slugs)
+        (host/blog--reach-down anchors)))))
 
 (define host/blog--relate-candidates
   (fn (slug q kind)
     (let ((spec (host/blog--kind-spec kind)))
-      (let ((pool (host/blog--candidate-pool (get spec :candidates)))
+      (let ((pool (host/blog--candidate-pool kind))
             (already (host/blog-out slug kind))
             (ql (lower (or q ""))))
         ;; pool is slugs; resolve titles, drop self + already-linked, filter by q