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.

tests/test_remote_pool.py

@@ -0,0 +1,52 @@
+"""Unit tests for the remote connection pool (browser_cli.remote.pool)."""
+import socket
+
+from browser_cli.remote import pool
+
+def _socketpair():
+  a, b = socket.socketpair()
+  return a, b
+
+def test_checkin_then_checkout_returns_same_connection():
+  pool.close_all()
+  a, b = _socketpair()
+  conn = pool.PooledConnection(a, b"secret")
+  pool.checkin("host:1", conn)
+  assert pool.checkout("host:1") is conn
+  assert pool.checkout("host:1") is None  # only one was pooled
+  b.close()
+  pool.close_all()
+
+def test_checkout_drops_stale_connection(monkeypatch):
+  pool.close_all()
+  a, b = _socketpair()
+  conn = pool.PooledConnection(a, b"secret")
+  pool.checkin("host:2", conn)
+  # Make the pooled connection look older than the idle bound.
+  conn.last_used -= (pool._MAX_IDLE_SECONDS + 1)
+  assert pool.checkout("host:2") is None  # stale → dropped, not returned
+  b.close()
+  pool.close_all()
+
+def test_checkin_caps_pool_size():
+  pool.close_all()
+  sockets = []
+  for i in range(pool._MAX_PER_ENDPOINT + 3):
+    a, b = _socketpair()
+    sockets.append(b)
+    pool.checkin("host:3", pool.PooledConnection(a, b"secret"))
+  drained = 0
+  while pool.checkout("host:3") is not None:
+    drained += 1
+  assert drained == pool._MAX_PER_ENDPOINT
+  for b in sockets:
+    b.close()
+  pool.close_all()
+
+def test_session_inner_message_strips_auth_fields():
+  msg = {
+    "id": "1", "command": "tabs.list", "args": {}, "user_agent": "browser-cli/1",
+    "pubkey": "x", "sig": "y", "pq_kex": {}, "encrypted": {}, "accept_encoding": {"x": 1},
+  }
+  inner = pool.session_inner_message(msg)
+  assert inner == {"id": "1", "command": "tabs.list", "args": {}, "user_agent": "browser-cli/1", "accept_encoding": {"x": 1}}