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>

web/tests/test-relate-picker.sx

@@ -136,6 +136,23 @@
         "</form>"
       "</li>")))
 
+;; One CURRENT-relation row, as host/blog--relation-editor renders it: an AJAX
+;; in-place remove (sx-post + sx-target=#cur-… + sx-swap=delete).
+(define cur-row-html
+  (fn (slug kind other label)
+    (str
+      "<li id=\"cur-" kind "-" other "\">"
+        "<a href=\"/" other "/\">" label "</a> "
+        "<form method=\"post\" action=\"/" slug "/unrelate\""
+          " sx-post=\"/" slug "/unrelate\""
+          " sx-target=\"#cur-" kind "-" other "\""
+          " sx-swap=\"delete\">"
+          "<input type=\"hidden\" name=\"other\" value=\"" other "\">"
+          "<input type=\"hidden\" name=\"kind\" value=\"" kind "\">"
+          "<button type=\"submit\">remove</button>"
+        "</form>"
+      "</li>")))
+
 ;; The "load more" sentinel, as host/blog--picker-more renders it.
 (define sentinel-html
   (fn (slug kind offset)
@@ -290,6 +307,44 @@
       (assert-equal 3 (count-candidates results))
       (clear-root! root))))
 
+;; ── regression: removing a relation must not clear the relate picker ─
+;; The remove button is an AJAX in-place delete (sx-post + sx-swap=delete on its own
+;; current-relation row). Submitting it deletes ONLY that row — the sibling picker's
+;; candidate list is left intact, because #content is never re-rendered. (Bug: the
+;; old plain-boosted remove redirected and the re-rendered picker came back empty.)
+(defsuite
+  "relate-picker:unrelate-keeps-picker"
+  (deftest
+    "removing a current relation deletes just that row, leaving the picker intact"
+    (reset-fetch-mock!)
+    ;; the AJAX unrelate returns an empty 200 (like the host); sx-swap=delete then
+    ;; removes the current-relation row in place.
+    (let
+      ((root (mk-root (str
+               "<div id=\"cur-box\"><ul>"
+                 (cur-row-html "host" "related" "beta" "Beta")
+                 (cur-row-html "host" "related" "gamma" "Gamma")
+               "</ul></div>"
+               "<div id=\"res-box\"><ul class=\"rp-results\">"
+                 (row-html "host" "related" "delta" "Picker Delta")
+                 (row-html "host" "related" "epsilon" "Picker Epsilon")
+               "</ul></div>")))
+        (cur-box (dom-query "#cur-box"))
+        (res-box (dom-query "#res-box")))
+      (process-elements root)
+      ;; two current relations, two picker candidates to start
+      (assert-equal 2 (len (dom-query-all cur-box "li")))
+      (assert-equal 2 (len (dom-query-all res-box "li")))
+      ;; remove Beta — submit its in-place remove form
+      (fire-event! (dom-query (dom-query "#cur-related-beta") "form") "submit")
+      ;; just Beta's row is gone; Gamma remains
+      (assert-nil (dom-query "#cur-related-beta"))
+      (assert-true (not (nil? (dom-query "#cur-related-gamma"))))
+      (assert-equal 1 (len (dom-query-all cur-box "li")))
+      ;; and the picker's candidate list is UNTOUCHED — the bug was it cleared to 0
+      (assert-equal 2 (len (dom-query-all res-box "li")))
+      (clear-root! root))))
+
 ;; ── Phase 3: the engine drives a non-browser target (the console) ───
 ;; render-to-console (web/console-render.sx) prints the live engine tree as text.
 ;; These assert the picker's terminal rendering directly on a built tree — the