test(e2e): fuse the scroll guard and the write it guards

`album-dropdown`'s "can be scrolled" failed twice over two sessions with
`Expected 80, Received 10`, both times on a branch that could not have
caused it. #133 strengthened the guard from "scrollable at all" to "has
the range this assertion needs", which was necessary and cannot be
sufficient: the guard and the write are separate round trips, so the
page re-lays-out between them.

Measured every frame across the resize, three runs: the range goes 0 →
**88** at 1ms → 330 settled by 8-14ms. 88 satisfies a guard asking for
80 while the grid is still a pass from done, so the guard is capable of
passing on a layout that is about to move. Under full-suite load the
transient is worse — the observed failures read 10 — which is why this
shows up on the second run of a suite and not in ten consecutive runs
of the file alone (0/10 before the change and after it; isolation is
not where this lives).

So the probe sets `scrollTop` and returns what it reads back, in one
page-side call, and the poll retries that. The assertion is now about
what the grid did rather than about what it was ready to do, and there
is no window between deciding and doing for anything to happen in.

#133's own last line asked for the other viewport-shrinking specs to be
swept for the same shape. One had it: `layout-overflow`'s sidebar probe
already fused its scroll and its measurement into one evaluate but ran
it once, so it read whatever the sidebar happened to be doing after the
resize. It is polled now — safe to repeat, because scrolling to the
bottom twice is scrolling to the bottom.

Closes #151

e2e/specs/layout-overflow.spec.ts

@@ -132,22 +132,34 @@ test.describe('the app fits in its own window', () => {
     // be dragged here, but a scaled display or a large system font can
     // still land the layout in it, and clipping the nav with no scroll
     // is the failure that made Settings unreachable.
-    const reachable = await app.locator('app-sidebar').evaluate((el) => {
-      const settings = el.shadowRoot?.querySelector<HTMLElement>(
-        '[data-testid="nav-settings"]',
-      );
+    //
+    // The scroll and the measurement share one `evaluate` — #151's
+    // rule, which this already had — and the whole probe is polled,
+    // which it did not: a viewport change settles asynchronously, so a
+    // single attempt reads whatever the sidebar happened to be doing.
+    // The probe is safe to repeat because scrolling to the bottom twice
+    // is scrolling to the bottom.
+    await expect
+      .poll(() =>
+        app.locator('app-sidebar').evaluate((el) => {
+          const settings = el.shadowRoot?.querySelector<HTMLElement>(
+            '[data-testid="nav-settings"]',
+          );
 
-      if (!settings) return null;
+          if (!settings) return null;
 
-      el.scrollTop = el.scrollHeight;
+          el.scrollTop = el.scrollHeight;
 
-      const item = settings.getBoundingClientRect();
-      const pane = el.getBoundingClientRect();
+          const item = settings.getBoundingClientRect();
+          const pane = el.getBoundingClientRect();
 
-      return item.bottom <= Math.ceil(pane.bottom) && item.top >= Math.floor(pane.top);
-    });
-
-    expect(reachable).toBe(true);
+          return (
+            item.bottom <= Math.ceil(pane.bottom) &&
+            item.top >= Math.floor(pane.top)
+          );
+        }),
+      )
+      .toBe(true);
   });
 });