host: relate/unrelate keep both lists in sync (add to current list, never blank the picker)

Two reported bugs on the edit page's relation editor:
 1. relating a candidate didn't add it to the current-relations list (the AJAX
    relate just deleted the candidate row; the relation only showed after a reload);
 2. removing a relation could blank the relate picker.

Fix (lib/host/blog.sx): both the candidate's relate form and a current relation's
remove form now target #rel-editor-<kind> with sx-swap=outerHTML, and the
relate/unrelate handlers return the re-rendered editor for that kind (current list +
a fresh picker). So one swap keeps BOTH lists in sync: the related post moves into
the current list and out of the (re-loaded) candidate pool; removing moves it back.
Gated on the SX-Target header, so a plain boosted form / no-JS POST (the is-a-tag
toggle) still redirects + re-renders #content.

Engine fix (web/orchestration.sx): handle-html-response's non-select branch called
post-swap on the OLD target, which an outerHTML swap has already REPLACED — so the
swapped-in content's triggers (here the re-rendered picker's "load") never bound and
the picker stayed empty. post-swap the swap result (the new node), mirroring the
sx-select branch. Recompiled orchestration.sxbc for the content-addressed client.

Tests:
 - web/tests/test-relate-picker.sx: relating re-syncs the editor (post in current
   list + picker re-loads); removing does likewise — both fail without the engine fix.
 - lib/host/tests/blog.sx: relate/unrelate return the re-rendered editor fragment
   (200, #rel-editor + picker), forms wire to #rel-editor-KIND/outerHTML, plain
   boosted POST still 303.
 - relate-picker.spec.js: the full in-page flow (relate adds to list, remove keeps
   the picker, no reload) + persistence.

Verified: host conformance 277/277, web engine suite 8/8, run-picker-check 3/3,
run-spa-check 3/3.

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

lib/host/playwright/relate-picker.spec.js

@@ -50,28 +50,53 @@ async function login(page) {
 }
 
 test.describe('relate picker (browser-only)', () => {
-  test('the remove button on a current relation actually unrelates it', async ({ page }) => {
+  test('relating a candidate adds it to the current list AND removing keeps the picker', async ({ page }) => {
+    // The whole in-page flow the user reported broken — no reloads. Relating a
+    // candidate re-renders the editor: the post moves into the current-relations
+    // list and the picker re-loads its candidates (it is NOT blanked). Removing it
+    // re-renders the editor back: the post leaves the current list and the picker
+    // still offers candidates.
     test.setTimeout(75000);
     await loginTo(page, `/${HOST}/edit`);
     await waitReady(page);
-    // relate Item 13 via the picker, then reload so it shows in the current list
+    await page.evaluate(() => { window.__noReload = true; });
+    // relate Item 13 from the picker
     await page.fill(RELF, 'Item 13');
     await expect.poll(() => page.locator(RELROWS).count(), { timeout: 10000 }).toBe(1);
     await page.locator(`${RELROWS} button`).first().click();
-    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 10000 }).toBe(0);
+    const relLink = page.locator('a[href="/picker-item-13/"]');
+    // ISSUE 1: it now appears in the CURRENT relations list (added, not just removed)
+    await expect(relLink).toHaveCount(1, { timeout: 12000 });
+    // and the re-rendered picker still offers candidates (not blanked)
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
+    // now remove it via its current-list remove button
+    await page.locator('li:has(a[href="/picker-item-13/"]) button').click();
+    await expect(relLink).toHaveCount(0, { timeout: 12000 });   // left the current list
+    // ISSUE 2: removing must NOT clear "the list of posts to relate"
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 12000 }).toBeGreaterThan(0);
+    expect(await page.evaluate(() => window.__noReload)).toBe(true);   // all in-page, no reload
+    // and the relation truly persisted gone (reload shows it not present)
     await page.reload();
     await waitReady(page);
-    const relLink = page.locator('a[href="/picker-item-13/"]');
-    await expect(relLink).toHaveCount(1);                  // current relation present
-    // 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);
+    await expect(page.locator('a[href="/picker-item-13/"]')).toHaveCount(0);
+  });
+
+  test('relating a candidate persists the relation', async ({ page }) => {
+    test.setTimeout(75000);
+    await loginTo(page, `/${HOST}/edit`);
+    await waitReady(page);
+    await page.fill(RELF, 'Item 07');
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 10000 }).toBe(1);
+    await page.locator(`${RELROWS} button`).first().click();
+    await expect(page.locator('a[href="/picker-item-07/"]')).toHaveCount(1, { timeout: 12000 });
+    // persisted across a reload
+    await page.reload();
+    await waitReady(page);
+    await expect(page.locator('a[href="/picker-item-07/"]')).toHaveCount(1);
+    // and visible on the public post page
+    await page.goto(`/${HOST}/`);
+    await expect(page.getByRole('heading', { name: 'Related posts' })).toBeVisible();
+    await expect(page.locator('body')).toContainText('Picker Item 07');
   });
 
   test('picker populates after a boosted SPA nav to the edit page', async ({ page }) => {