feat: configurable Metro port

[mfazekas]

Summary

Adds a metroPort config option (defaults to 8081) so harness tests can run on a non-default Metro port. This enables running independent harness sessions across git worktrees and avoids conflicts when port 8081 is already in use.

• iOS simulator: sets RCT_jsLocation via xcrun simctl spawn ... defaults write
• Android: writes debug_http_host SharedPreferences via adb shell run-as + sets up adb reverse for the configured port
• iOS physical devices: throws if non-default port is configured (no remote NSUserDefaults access)
• Always writes the port preference on startup (even for default 8081) to recover from stale state after crashes

Usage

// rn-harness.config.mjs
export default {
  metroPort: 8082,
  // ...
};

Or via CLI:

npx jest --config jest.harness.config.mjs --metroPort 8082

Limitations

• Parallel worktrees must use different simulators/emulators — the port preference is per-bundle-ID per-device, so two sessions targeting the same device would race.
• iOS physical devices don't support custom Metro ports (no way to set RCT_jsLocation remotely).

Test plan

• Typecheck passes across all 20 packages
• All unit tests pass (48 tests)
• iOS simulator: smoke test passes with metroPort: 8082 while 8081 is occupied
• Android emulator: smoke test passes with metroPort: 8082 while 8081 is occupied
• Default port (8081) correctly fails with Metro port 8081 is not available when occupied

[vercel]

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[V3RON]

Hey @mfazekas 👋
I went ahead and pushed a few changes so --metroPort now ends up in HarnessConfig, and the changes to shared prefs and iOS defaults are reverted after the run. Let me know what you think.

[mfazekas]

@V3RON thanks much! Yes both changes make sense to me 👍