feat: harden remote serve and reuse connections

- Gate TCP serve commands with safe-by-default policies, per-key allow tokens, per-key rate limiting, and audit labels.
- Reuse authenticated encrypted remote sessions and parallelize/caches multi-browser fanout to reduce repeated handshake roundtrips.
- Increase paged native-host batch size with extension-side byte budgeting to speed large tab listings safely.
- Point install output at public Chrome Web Store / Firefox AMO listings by default, with --dev preserving unpacked workflows.
- Share search-engine metadata between CLI and SDK and bump the package/extension version to 0.16.0.
- Cover the new security, pooling, paging, install, and fanout behavior with expanded Python and extension tests.

browser_cli/sdk/navigation.py

@@ -114,7 +114,7 @@ class NavigationNS(Namespace):
   ) -> None:
     """Open a search query in the given engine (e.g. 'google', 'youtube', 'ddg')."""
     from urllib.parse import quote_plus
-    from browser_cli.commands.search import ENGINES
+    from browser_cli.search.engines import ENGINES
     template = ENGINES.get(engine)
     if template is None:
       raise ValueError(f"Unknown search engine '{engine}'. Available: {', '.join(ENGINES)}")