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)}")

browser_cli/sdk/routing.py

@@ -7,12 +7,14 @@ helpers; single-browser mode falls straight through to ``_cmd``.
 """
 from __future__ import annotations
 
+import asyncio
 import importlib
 import sys
 from collections.abc import Callable, Iterable
 from typing import TYPE_CHECKING, Protocol, cast
 
 from browser_cli.client import BrowserTarget
+from browser_cli.client.core import _run_concurrent
 from browser_cli.errors import BrowserNotConnected
 from browser_cli.models import BrowserCounts, Tab
 
@@ -81,18 +83,28 @@ class RoutingMixin:
     return targets
 
   def _collect_multi_browser(self, command: str, args: dict | None = None):
-    results = []
     targets = self._multi_browser_targets()
-    for target in targets:
-      try:
-        if target.remote:
-          data = _browser_cli_package().send_command(
-            command, args, profile=target.profile, remote=target.remote, key=self._client._key
-          )
-        else:
-          data = _browser_cli_package().send_command(command, args, profile=target.profile)
-      except (BrowserNotConnected, RuntimeError):
+
+    def _send(target: BrowserTarget):
+      package = _browser_cli_package()
+      if target.remote:
+        return package.send_command(
+          command, args, profile=target.profile, remote=target.remote, key=self._client._key
+        )
+      return package.send_command(command, args, profile=target.profile)
+
+    # Run per-target roundtrips concurrently — each is a blocking, network-bound
+    # send_command, so offloading to threads gives real overlap while still
+    # invoking the (test-patchable) sync entry point.
+    raw = _run_concurrent([
+      (lambda t=t: asyncio.to_thread(_send, t)) for t in targets
+    ])
+    results = []
+    for target, data in zip(targets, raw):
+      if isinstance(data, (BrowserNotConnected, RuntimeError)):
         continue
+      if isinstance(data, BaseException):
+        raise data
       results.append((target, data))
     if results:
       return results