host: relations-as-posts slice 2.5 — picker title reads are O(page), not O(pool)

relate-candidates computes the available candidate SLUGS (slug-sorted, no per-candidate
read), then reads titles only for the page it returns. On the unfiltered path (q="" —
the initial picker load AND every editor server-fill, the common case) that's ~limit
durable reads instead of one-per-post, cutting the http-listen suspend/resume churn. A
filter (q≠"") still resolves titles across the pool since it matches on the title.

(A boot slug→title cache would make the filter O(1)-perform too, but it's blocked: no
bulk KV read, and a per-post host/blog-get loop at boot hits the JIT 'durable read in a
boot loop drops all-but-first' bug — see plans/relations-as-posts.md.)

conformance 291/291, run-picker-check 3/3 (incl. the title filter + paging).

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

lib/host/blog.sx

@@ -418,24 +418,34 @@
   (fn (kind other)
     (contains? (host/blog--candidate-pool kind) other)))
 
+(define host/blog--title (fn (s) (get (host/blog-get s) :title)))   ;; one durable read
+
+;; One PAGE of candidates (records {:slug :title}) for relating `slug` under `kind`.
+;; Slice 2.5 — title reads are O(page), not O(pool): the available candidate SLUGS are
+;; computed + slug-sorted with NO per-candidate read; then titles are fetched only for
+;; the rows actually returned. On the unfiltered path (q="" — the initial picker load
+;; AND every editor server-fill) that's ~`limit` reads instead of one-per-post, which
+;; was the durable-read churn under http-listen. A filter (q≠"") still resolves titles
+;; across the pool, since it matches on the title — but that's the interactive path.
 (define host/blog--relate-candidates
-  (fn (slug q kind)
-    (let ((spec (host/blog--kind-spec kind)))
-      (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
-        (let ((cands
-                (filter
-                  (fn (p)
-                    (or (= ql "")
-                        (contains? (lower (get p :title)) ql)
-                        (contains? (get p :slug) ql)))
-                  (map (fn (s) {:slug s :title (get (host/blog-get s) :title)})
-                    (filter (fn (s) (and (not (= s slug)) (not (contains? already s)))) pool)))))
-          ;; title-sort via [title slug] pairs (sort compares the title first)
-          (map (fn (pair) {:slug (nth pair 1) :title (nth pair 0)})
-            (sort (map (fn (p) (list (get p :title) (get p :slug))) cands))))))))
+  (fn (slug q kind offset limit)
+    (let ((pool (host/blog--candidate-pool kind))
+          (already (host/blog-out slug kind))
+          (ql (lower (or q ""))))
+      (let ((avail (sort (filter (fn (s) (and (not (= s slug)) (not (contains? already s)))) pool))))
+        (if (= ql "")
+          ;; no filter: page by slug, then read titles for just the page
+          (map (fn (s) {:slug s :title (host/blog--title s)})
+            (take (drop avail offset) limit))
+          ;; filter: resolve titles, match on title|slug, then page
+          (let ((recs (map (fn (s) {:slug s :title (host/blog--title s)}) avail)))
+            (take
+              (drop
+                (filter (fn (r) (or (contains? (lower (get r :title)) ql)
+                                    (contains? (get r :slug) ql)))
+                  recs)
+                offset)
+              limit)))))))
 
 ;; One candidate row: a tiny form whose button adds the relation under `kind`.
 (define host/blog--picker-item
@@ -492,8 +502,7 @@
           ;; so a filter like "Item 13" arrives as "Item%2013" — decode it.
           (q (dr/url-decode (or (dream-query-param req "q") "")))
           (offset (host/query-int req "offset" 0)))
-      (let ((page (take (drop (host/blog--relate-candidates slug q kind) offset)
-                    host/blog--picker-limit)))
+      (let ((page (host/blog--relate-candidates slug q kind offset host/blog--picker-limit)))
         (let ((rows (join "" (map (fn (p) (render-page (host/blog--picker-item slug p kind))) page)))
               (more (if (= (len page) host/blog--picker-limit)
                       (render-page (host/blog--picker-more slug kind q (+ offset host/blog--picker-limit)))
@@ -686,8 +695,7 @@
           ;; li-trees splice in as children (component args would evaluate them).
           (results-ul
             (let ((rows (if with-cands
-                          (let ((cands (take (host/blog--relate-candidates slug "" kind)
-                                         host/blog--picker-limit)))
+                          (let ((cands (host/blog--relate-candidates slug "" kind 0 host/blog--picker-limit)))
                             (append
                               (map (fn (p) (host/blog--picker-item slug p kind)) cands)
                               (if (= (len cands) host/blog--picker-limit)