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/tests/blog.sx

@@ -295,6 +295,37 @@
 (host-bl-test "relate-options omits the load-more sentinel on a short last page"
   (contains? (dream-resp-body (host-bl-app (host-bl-req "/alpha-post/relate-options"))) "rp-more")
   false)
+
+;; -- unrelate: AJAX in-place row delete (regression: removing a related post must
+;;    NOT clear the relate picker). The remove button is now an sx-post form that
+;;    deletes just its own current-relation row (sx-target=#cur-…, sx-swap=delete),
+;;    so #content is never re-rendered and the picker is left intact. --
+(host/blog-relate! "alpha-post" "beta-post" "related")
+(host-bl-test "relation-editor remove button is an AJAX in-place delete"
+  (let ((html (render-page (host/blog--relation-editor "alpha-post" "related"))))
+    (list (contains? html "id=\"cur-related-beta-post\"")        ;; row has a target id
+          (contains? html "sx-post=\"/alpha-post/unrelate\"")    ;; AJAX, not plain post
+          (contains? html "sx-target=\"#cur-related-beta-post\"")
+          (contains? html "sx-swap=\"delete\"")))
+  (list true true true true))
+;; the AJAX remove (carries SX-Target) returns an empty 200 so only the row is
+;; swapped out — no redirect, no #content re-render that would blank the picker.
+(host-bl-test "unrelate (AJAX, SX-Target) returns an empty 200"
+  (let ((resp (host/blog-unrelate-submit
+                (dream-request "POST" "/alpha-post/unrelate"
+                  {:sx-request "true" :sx-target "#cur-related-beta-post"}
+                  "other=beta-post&kind=related"))))
+    (list (dream-status resp) (dream-resp-body resp)))
+  (list 200 ""))
+;; a plain boosted form / no-JS POST (no SX-Target) still redirects + re-renders,
+;; so the is-a-tag toggle and graceful degradation are unaffected.
+(host/blog-relate! "alpha-post" "beta-post" "related")
+(host-bl-test "unrelate (plain boosted / no-JS, no SX-Target) still redirects"
+  (dream-status (host/blog-unrelate-submit
+                  (dream-request "POST" "/alpha-post/unrelate"
+                    {:sx-request "true"} "other=beta-post&kind=related")))
+  303)
+(host/blog-unrelate! "alpha-post" "beta-post" "related")
 (host/blog-put! "hint-post" "Hint Post" "(p \"h\")" "published")
 (host-bl-test "relations section: hint when logged-in + no relations"
   (contains? (str (host/blog--relations-or-hint "hint-post" true)) "add some") true)