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/client/targets.py

@@ -19,6 +19,11 @@ class BrowserTarget:
   remote: str | None = None
   browser_name: str | None = None
   display_group: str | None = None
+  # Populated from a remote ``browser-cli.targets`` response when the remote is
+  # new enough to advertise them, letting ``clients`` skip a redundant
+  # ``clients.list`` roundtrip. None means "unknown — fall back to a query".
+  version: str | None = None
+  extension_version: str | None = None
 
 def is_reachable_unix_endpoint(endpoint: str) -> bool:
   """Return True when a Unix socket path exists and accepts connections."""