refactor: reorganize test structure into unit, integration, performance, and e2e

- Move tests from root test/ to organized subdirectories
- Extract edge cases to appropriate test files
- Remove duplicate and DOM-only tests
- Add test-plan.md for documentation
- Total tests: 202 (previously 207 with duplicates)

Author: aaronlippold

test/basic.test.ts renamed to test/e2e/basic.test.ts

@@ -4,7 +4,7 @@ import { setup, $fetch } from '@nuxt/test-utils/e2e'
 
 describe('ssr', async () => {
   await setup({
-    rootDir: fileURLToPath(new URL('./fixtures/basic', import.meta.url)),
+    rootDir: fileURLToPath(new URL('../fixtures/basic', import.meta.url)),
   })
 
   it('renders the index page', async () => {