[KS] Driver dev builds: source maps, /symbolicate, prebundled worker entry (#408)

[sethwebster]

Refs #408

What changed

• scripts/e2e/bundle-server.ts (new): the driver's bundle server for dev builds. It serves /index.bundle and /.threaded-runtime/entry.bundle, their source maps (/index.map, /.threaded-runtime/entry.map), and Metro's POST /symbolicate. Like Metro's server, it rewrites each bundle's trailing sourceURL/sourceMappingURL to the URL that was actually served. export:embed writes a http://localhost:8081/... sourceURL, which nothing serves, and Hermes names profile frames by it.
• Driver runs (DriverBackend) prebundle the worker entry in the background once the app's bundle is built. A request still bundles the entry lazily if the prebundle hasn't finished or failed. Failures go to stderr and are never silent. A failed build answers 500 and is retried.
• exportBundle writes source maps. scripts/testing/source-map.ts now indexes by line, returns source columns, and treats unmapped segments as unmapped.
• e2e/dev-flows/devtools/profiling-dev.yaml checks that the ks-devtools worker runs, writes isolated-ks-devtools.cpuprofile, and that both runtimes report source maps applied.
• The first-worker waits in plugins-load, plugins-unload, isolated-runtime and profiling-dev drop from 60 s to 30 s. With a warm Metro cache, the dev worker entry took 5 s to bundle (the app bundle 11 s) when measured on this Mac.
• Fixes found while running the flows:
  • KitchenSinkApp.open() now closes the bundle server when the session fails to open. Before, the runner never closed it and the run hung after its summary.
  • Clearing driver state now leaves dirhelper's protected TemporaryItems. Every operation on it fails with EPERM, so every clearState flow was failing.
• Docs: docs/e2e-flows.md (the driver backend) and docs/devtools.md (Verification).

Verification (run in this session)

• bun run typecheck: passes.
• bun run e2e:lint: 0 problems.
• bun run test: 2008 passed, 2 failed. Both failures are the accepted ones: node-runtime (asdf shim) and motion-native frames (display asleep).
• New tests/e2e-bundle-server.test.ts (7 tests). tests/e2e-app-state.test.ts and tests/e2e-driver-backend.test.ts were updated.
• ks-lock native bun run e2e:run --backend driver on a dev build (rebuild:macos): plugins-load/unload/rejections/visual, multiwindow/isolated-runtime, infra/catalog-driver, dev-flows/devtools/profiling-dev, dev-flows/devtools/react-devtools-dev: 14 passed, 0 failed. plugins-load ran first, so it covered the first-worker case.
• Same flows except the dev-only ones on the release build (bun run build, --app .../release/KitchenSink.app): 12 passed, 0 failed.