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/playwright/relate-picker.spec.js

@@ -63,10 +63,15 @@ test.describe('relate picker (browser-only)', () => {
     await waitReady(page);
     const relLink = page.locator('a[href="/picker-item-13/"]');
     await expect(relLink).toHaveCount(1);                  // current relation present
-    // click its remove button — a plain boosted form (regression: this did nothing
-    // because bind-boost-form discarded the form's method/action)
+    // the picker is populated (empty filter -> first page of candidates)
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
+    // click its remove button
     await page.locator('li:has(a[href="/picker-item-13/"]) button').click();
     await expect(relLink).toHaveCount(0, { timeout: 12000 });   // relation removed
+    // REGRESSION: removing a relation must NOT clear "the list of posts to relate".
+    // (The old plain-boosted remove form redirected -> re-rendered #content and the
+    // picker came back empty; the AJAX in-place remove leaves the picker untouched.)
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
   });
 
   test('picker populates after a boosted SPA nav to the edit page', async ({ page }) => {