test(e2e): make the visual gate actually sensitive, and fix its masking race

The suite was carrying Playwright's 0.2 per-pixel default, a tolerance meant for
cross-machine rendering noise that the pinned container removes by construction.
It cost the gate its sensitivity: a real palette change passed while three
baselines silently went stale.

Compare strictly and bound the differing-pixel count instead. Measured over
repeated container runs, glyph antialiasing jitters by exactly 2 pixels, while the
palette change that slipped through moved 71 — so threshold 0 with a 20-pixel
allowance tolerates the noise and catches the class of change that got away.

Strict comparison also exposed a real defect the loose one hid: the flow detail's
ready locator was the header button, but the visual spec masks the terminal, which
mounts later. Capturing in that window compared live terminal pixels against a
masked baseline — a 261k-pixel diff, roughly a third of the frame, appearing at
random. The route now counts as rendered when its terminal exists.

Seven consecutive container runs are clean at the new settings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/e2e/playwright.config.ts

@@ -36,6 +36,11 @@ if (tier === 'stand' && !TIERS.stand.baseURL) {
 }
 
 export default defineConfig<BackendOptions>({
+    // Playwright's 0.2 per-pixel default absorbs cross-machine rendering noise the
+    // pinned container removes by construction; it also let a palette change through
+    // while reporting green. Count differing pixels instead: glyph antialiasing
+    // jitters by 2, while the palette change that slipped moved 71.
+    expect: { toHaveScreenshot: { maxDiffPixels: 20, threshold: 0 } },
     forbidOnly: isCI,
     fullyParallel: true,
     globalTimeout: isCI ? 10 * 60_000 : undefined,

frontend/e2e/routes.ts

@@ -44,7 +44,9 @@ export const ROUTE_MANIFEST: RouteManifestEntry[] = [
     {
         cassette: flowsCassette,
         path: routes.flow('5'),
-        ready: (page) => page.getByRole('button', { name: 'Flow actions' }),
+        // The terminal mounts after the header, and the visual spec masks it —
+        // capturing before it exists compares live pixels against a masked baseline.
+        ready: (page) => page.locator('.xterm').first(),
         sources: [
             'src/pages/flows',
             'src/features/flows',