[codex] support React Router 8 compatibility

[ScriptedAlchemy]

Summary

• Add examples/react-router-8 as a direct copy of React Router’s integration/helpers/vite-8-template, with the Vite config/scripts swapped for this Rsbuild plugin.
• Add Playwright dev and production e2e coverage for the copied React Router 8 default template, including browser console/page-error/request-failure checks.
• Resolve RR8 hydration/runtime compatibility by using React Router’s installed UNSAFE_* route wrapper helpers and aliasing react-router package imports to the app install.
• Include the RR8 example in the repo e2e example filter and document it in the examples table.
• Remove the stale packed version-matrix script/command and consolidate duplicate changesets into one RR8 compatibility changeset.

Validation

• pnpm prettier --write package.json README.md src/index.ts src/plugin-utils.ts src/route-component-transform.ts tests/index.test.ts tests/plugin-utils.test.ts examples/react-router-8/package.json examples/react-router-8/README.md examples/react-router-8/app/root.tsx examples/react-router-8/app/routes.ts examples/react-router-8/app/routes/_index.tsx examples/react-router-8/env.d.ts examples/react-router-8/playwright.config.ts examples/react-router-8/react-router.config.ts examples/react-router-8/rsbuild.config.ts examples/react-router-8/tests/e2e/react-router-8.test.ts examples/react-router-8/tsconfig.json
• pnpm prettier --write package.json .changeset/react-router-8-default-template.md
• git diff --check
• pnpm test:core -- tests/plugin-utils.test.ts tests/index.test.ts
• pnpm test:core -- tests/plugin-utils.test.ts tests/index.test.ts tests/prerender.test.ts tests/react-router-config.test.ts tests/modify-browser-manifest.test.ts
• pnpm --filter react-router-8-default-template typecheck
• pnpm build
• pnpm --filter react-router-8-default-template test:e2e
• pnpm test:package-interop

TraceDecay was used for changed-file impact context; latest diff context mapped the plugin changes to route transform/dev runtime coverage and affected tests.

Note: this PR is stacked on codex/prewarm-route-workers because the worktree started from that branch. Against that base, this branch contains the React Router 8 compatibility/example work.

[github-actions]

Benchmark Results

Compared PR head a8cb0f2 against base 6c6000b.

Total median wall time: 9.79s -> 9.72s (-0.7%, 1.01x speedup)
Compiler ready median: 9.79s -> 9.72s (-0.7%)
Route load median: 2.01s -> 1.97s (-2.0%)

Benchmark	Base total	Head total	Delta	Head ready	Head routes	Speedup	Head RSS p95
large-355-ssr-esm	9.79s	9.72s	-0.7%	9.72s	1.97s	1.01x	-

Profile: large; mode: dev; iterations: 10; warmup: 0.
Workflow run