host: relate removes just the picked candidate row in place (no reload)

Picking a candidate to relate it no longer does a full POST -> 303 -> reload.
The candidate <li> now carries an id and its relate form is an AJAX sx-post
(sx-target="#cand-<kind>-<other>", sx-swap="delete"): on success the engine
deletes just that one row — the item is now related, so it leaves the candidate
pool with no reload and no candidate-list refetch. host/blog-relate-submit returns
an empty 200 for an SX request (so the delete swap fires) and still 303s for a
plain POST (no-JS fallback via the form's method+action).

relate-picker.spec.js test 4 updated to assert the in-place row delete + no reload
+ the relation still persists (shows on the post page). 6/6 + conformance 272/272.

(Symmetric unrelate-in-place was prototyped but backed out: the current-links
form, bound via boot's process-elements rather than post-swap, didn't fire the
AJAX delete despite identical markup — a binding quirk to chase separately. Unrelate
keeps its plain POST -> reload for now, no regression.)

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

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

@@ -78,19 +78,20 @@ test.describe('relate picker', () => {
     await expect(page.locator(RELR)).toContainText('Picker Item 13');
   });
 
-  test('clicking a candidate relates it (and it shows on the post page)', async ({ page }) => {
-    // heavier than the others: a full-reload relate (sx-disable) then a goto to the
-    // post page — two WASM kernel boots — so it needs more than the 30s default.
+  test('clicking a candidate relates it and removes just that row (no reload)', async ({ page }) => {
     test.setTimeout(75000);
     await loginTo(page, `/${HOST}/edit`);
     await waitReady(page);
     await page.fill(RELF, 'Item 07');
     await expect.poll(() => page.locator(RELROWS).count(), { timeout: 10000 }).toBe(1);
+    // sentinel survives ONLY if there is no full-page reload
+    await page.evaluate(() => { window.__noReload = true; });
     await page.locator(`${RELROWS} button`).first().click();
-    // form POST -> 303 back to the edit page; the related list now links the slug
-    await expect(page).toHaveURL(new RegExp(`/${HOST}/edit`));
-    await expect(page.locator('a[href="/picker-item-07/"]')).toHaveCount(1);
-    // and the public post page shows the Related posts block with the title
+    // the AJAX relate deletes just that candidate row in place — no reload, no
+    // candidate-list refetch
+    await expect.poll(() => page.locator(RELROWS).count(), { timeout: 10000 }).toBe(0);
+    expect(await page.evaluate(() => window.__noReload)).toBe(true);
+    // and the relation actually persisted (shows on the public post page)
     await page.goto(`/${HOST}/`);
     await expect(page.getByRole('heading', { name: 'Related posts' })).toBeVisible();
     await expect(page.locator('body')).toContainText('Picker Item 07');