feat(voyage): add Playwright + @axe-core/playwright devDependencies

Step 27 of v4.3 plan — adds Playwright 1.59.1 + axe-core/playwright 4.10.2
as devDependencies, npm script test:e2e, and playwright.config.mjs with
file:// baseURL pointing at playground/. Chromium browser binary installed
to ~/Library/Caches/ms-playwright/.

Trace: SC2 (WCAG verifikasjon krever browser-driver), research/04
Recommendation security; brief Constraint linje 64 (zero new deps i
playground/) er OK fordi Playwright er devDependency på voyage-plugin-rot,
ikke i playground/.

plugins/voyage/playwright.config.mjs

@@ -0,0 +1,22 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: 'tests/e2e',
+  testMatch: '**/*.spec.mjs',
+  timeout: 30_000,
+  expect: { timeout: 5_000 },
+  fullyParallel: false,
+  forbidOnly: !!process.env.CI,
+  retries: 0,
+  reporter: process.env.CI ? 'github' : 'list',
+  use: {
+    baseURL: `file://${import.meta.dirname}/playground/`,
+    trace: 'retain-on-failure',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+  ],
+});