web: re-boost swapped content from the [sx-boost] ancestor (fixes back-then-click full reload)

After a fragment swap, process-elements(target) -> process-boosted(target) only
boosted [sx-boost] containers that are DESCENDANTS of the swap target. But the
swap target (#content) is nested UNDER the boost wrapper (<div sx-boost="#content">
<div id="content">), so re-boosting scoped to the target found nothing — the
swapped-in links never got bound. Only the initial document-wide boot boost
worked, so: home->sub worked (home links boosted at boot), but Back restored the
home content unboosted, and the next click did a full page reload. (Post-page
links were unboosted too; Back just exposed it.)

process-boosted now ALSO boosts from the nearest [sx-boost] ANCESTOR of root
(dom-closest), so any swap target inside a boost scope gets its links rebound.
is-processed? guards keep it idempotent.

spa-check: the back-button test now clicks AGAIN after Back and asserts it's a
SPA nav (no full reload) — would have caught this. .sxbc regenerated.

Verified: spa-check 4/4 (incl. click-after-back).

lib/host/playwright/spa-check.spec.js

@@ -67,5 +67,14 @@ test.describe('blog SPA', () => {
     await page.goBack();
     await page.waitForURL((u) => u.pathname === '/', { timeout: 15000 });
     await expect(page.locator('#content h1')).toContainText('Posts');
+    // and a click AFTER back must still be a SPA nav, not a full reload — the
+    // restored content has to be re-boosted (its [sx-boost] marker is an
+    // ancestor of the swap target, so the re-boost must scan upward).
+    await page.evaluate(() => { window.__noReload2 = true; });
+    const link2 = page.locator(POSTLINK).first();
+    const href2 = await link2.getAttribute('href');
+    await link2.click();
+    await page.waitForURL((u) => u.pathname === href2, { timeout: 15000 });
+    expect(await page.evaluate(() => window.__noReload2)).toBe(true);
   });
 });