ci: run frontend coverage in four shards (#565)

* ci: run frontend coverage in four shards

Coverage shards reached their 18-minute step limit three times on
2026-10-31 with every test passing. Split the run into four shards with
the limits unchanged; the merge job now expects four raw coverage
artifacts, and the vendor checks move to shard 4.

Closes #563

* test(tags): pin the 390px window check to the first page's last row

Reaching the last row loads the next page (D183), and last() then followed
row 200, far outside the window, so the WebKit check failed in 4-5 of 20
runs on 2 CPUs. Pinned to row 100, it passed 90 of 90 across WebKit,
Chromium and Firefox.

Closes #566

tests/browser/workspace-layout-tags-system.spec.mjs

@@ -156,8 +156,9 @@ test('at 390px the window fits the screen, actions stay reachable and the page s
     expect(frame.bottom-frame.top).toBeLessThan(844);
     expect(frame.bottom-frame.top).toBeGreaterThanOrEqual(280);
     expect(await page.evaluate(()=>document.documentElement.scrollWidth)).toBeLessThanOrEqual(390);
-    // The last row's actions can be scrolled to inside the window and fit its width.
-    const last=rows(page).last().getByRole('button',{name:'Rename / merge',exact:true});
+    // The first page's last row's actions can be scrolled to inside the window and fit its width.
+    // Pinned to row 100: reaching it loads the next page (D183), and last() would then follow row 200.
+    const last=rows(page).nth(99).getByRole('button',{name:'Rename / merge',exact:true});
     await last.scrollIntoViewIfNeeded();await expect(last).toBeInViewport();
     const [button]=await boxes(last);const [after]=await boxes(tagWindow(page));
     expect(button.right).toBeLessThanOrEqual(after.right);

tests/vendor/ci-workflow.test.mjs

@@ -129,11 +129,11 @@ test('Firefox/WebKit page workflows run only outside pull requests; coverage run
     }
 });
 
-test('coverage runs as three shards that each upload their raw coverage', () => {
+test('coverage runs as four shards that each upload their raw coverage', () => {
     const coverage = allJobs.get('frontend-coverage');
-    assert.deepEqual(matrixList(coverage, 'shard'), ['1', '2', '3']);
-    assert.match(coverage, /^ {4}name: frontend coverage \(shard \$\{\{ matrix\.shard \}\}\/3\)$/m);
-    assert.match(coverage, /npm run --prefix tools\/vendor test:coverage:shard -- \$\{\{ matrix\.shard \}\}\/3/);
+    assert.deepEqual(matrixList(coverage, 'shard'), ['1', '2', '3', '4']);
+    assert.match(coverage, /^ {4}name: frontend coverage \(shard \$\{\{ matrix\.shard \}\}\/4\)$/m);
+    assert.match(coverage, /npm run --prefix tools\/vendor test:coverage:shard -- \$\{\{ matrix\.shard \}\}\/4/);
     const upload = steps(coverage).find(step => /name: coverage-raw-\$\{\{ matrix\.shard \}\}/.test(step));
     assert.ok(upload, 'each shard uploads coverage-raw-N');
     assert.match(upload, /^ {10}path: coverage\/js\/raw\/?$/m);