refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

tests/test_remote_protocol_matrix.py

@@ -62,7 +62,7 @@ def no_browser(monkeypatch):
     def _raise_no_browser(*_args, **_kwargs):
         raise BrowserNotConnected("no browser")
 
-    monkeypatch.setattr("browser_cli.client._resolve_socket", _raise_no_browser)
+    monkeypatch.setattr("browser_cli.client.targets.resolve_socket", _raise_no_browser)
 
 def _connect(auth_keys_path):
     client, server = socket.socketpair()