host: removing a related post no longer clears the relate picker

Bug: the edit page's remove button (on a current relation) was a plain boosted
form — POST /unrelate -> 303 redirect -> the engine re-rendered #content, and the
freshly-swapped relate picker came back EMPTY ("the list of posts to relate" was
cleared).

Fix: make the remove button an AJAX in-place delete, exactly like the relate
candidate rows — each current-relation <li> gets an id and its form carries
sx-post + sx-target=#cur-<kind>-<other> + sx-swap=delete. unrelate-submit returns
an empty 200 for that request so the engine deletes just that one row; #content is
never re-rendered, so the picker is untouched. method+action stay for no-JS.

The empty-200 is gated on the SX-Target header (sent only by the sx-post form), so
a plain boosted form / no-JS POST still redirects + re-renders — the is-a-tag
toggle and graceful degradation are unaffected.

Tests (all red before the fix):
 - lib/host/playwright/relate-picker.spec.js: the remove-button test now asserts
   the picker still has candidates after a removal (the reproduction).
 - web/tests/test-relate-picker.sx: an SX engine test — removing a current relation
   deletes just that row and leaves the sibling picker's list intact.
 - lib/host/tests/blog.sx: the relation-editor renders the AJAX delete attrs;
   unrelate returns empty-200 with SX-Target and 303 without.

Verified: host conformance 275/275, web engine suite 8/8, run-picker-check 2/2.

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

lib/host/blog.sx

@@ -434,6 +434,15 @@
 ;; so the blog degrades gracefully to plain server-rendered pages.
 (define host/blog--spa-req? (fn (req) (= (dream-header req "sx-request") "true")))
 
+;; An AJAX in-place row delete vs. a plain boosted form. The engine sends an
+;; SX-Target header for an sx-post form (the relation-editor's remove button,
+;; sx-target=#cur-…), but NOT for a plain boosted form (the is-a-tag toggle). So
+;; this tells "delete just this row, empty 200" apart from "redirect + re-render".
+(define host/blog--row-swap-req?
+  (fn (req)
+    (and (host/blog--spa-req? req)
+         (let ((t (dream-header req "sx-target"))) (and t (not (= t "")))))))
+
 (define host/blog--page
   (fn (req title body)
     (if (host/blog--spa-req? req)
@@ -589,9 +598,18 @@
               (cons (quote ul)
                 (map (fn (s)
                        (quasiquote
-                         (li (a :href (unquote (str "/" s "/")) (unquote s)) " "
+                         ;; AJAX unrelate: sx-post the removal, then sx-swap="delete"
+                         ;; removes just THIS current-relation row (its sx-target) in
+                         ;; place — NO #content re-render, so the relate picker (the
+                         ;; "posts to relate" list) is left untouched. method+action
+                         ;; stay for the no-JS fallback (plain POST -> 303 -> reload).
+                         (li :id (unquote (str "cur-" kind "-" s))
+                           (a :href (unquote (str "/" s "/")) (unquote s)) " "
                            (form :method "post" :style "display:inline"
                              :action (unquote (str "/" slug "/unrelate"))
+                             :sx-post (unquote (str "/" slug "/unrelate"))
+                             :sx-target (unquote (str "#cur-" kind "-" s))
+                             :sx-swap "delete"
                              (input :type "hidden" :name "other" :value (unquote s))
                              (input :type "hidden" :name "kind" :value (unquote kind))
                              (button :type "submit" "remove")))))
@@ -879,7 +897,7 @@
             (dream-redirect (str "/" slug "/edit"))))))))
 
 ;; POST /<slug>/unrelate — remove the relation to `other` under `kind` (default
-;; "related"). Idempotent; redirects back to the edit page.
+;; "related"). Idempotent.
 (define host/blog-unrelate-submit
   (fn (req)
     (let ((slug (dream-param req "slug"))
@@ -888,7 +906,13 @@
       (begin
         (when (and other (not (= other "")) (host/blog--kind-spec kind))
           (host/blog-unrelate! slug other kind))
-        (dream-redirect (str "/" slug "/edit"))))))
+        ;; AJAX in-place remove (the editor's sx-post, carries SX-Target): return an
+        ;; empty 200 so the current-relation row's sx-swap="delete" removes just that
+        ;; one row — no #content re-render, so the relate picker is untouched. A plain
+        ;; boosted form (the tag toggle) or a no-JS POST still redirects + re-renders.
+        (if (host/blog--row-swap-req? req)
+          (dream-html "")
+          (dream-redirect (str "/" slug "/edit")))))))
 
 ;; GET /<slug>/edit — edit form pre-filled with the post's current title, raw
 ;; sx_content (in a textarea — render-to-html escapes the text child, so the