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.

browser_cli/client/__init__.py

@@ -0,0 +1,47 @@
+"""Client-side command routing and BrowserTarget helpers."""
+from browser_cli.client.targets import REGISTRY_PATH, is_active_local_profile, resolve_socket
+from browser_cli.client.core import (
+  BrowserNotConnected,
+  BrowserTarget,
+  _remote_browser_targets,
+  _send_remote,
+  _send_remote_async,
+  active_browser_targets,
+  remote_browser_targets,
+  remote_browser_targets_async,
+  remote_target_for_alias,
+  send_command,
+  send_command_async,
+)
+from browser_cli.endpoints import (
+  _looks_like_domain,
+  _normalize_endpoint,
+  _remote_display_name,
+  _resolve_connect_endpoint,
+  display_browser_name,
+)
+from browser_cli.remote.transport import _recv_all, _recv_exact
+
+__all__ = [
+  "BrowserNotConnected",
+  "BrowserTarget",
+  "REGISTRY_PATH",
+  "is_active_local_profile",
+  "_looks_like_domain",
+  "_normalize_endpoint",
+  "_recv_all",
+  "_recv_exact",
+  "_remote_browser_targets",
+  "_remote_display_name",
+  "_resolve_connect_endpoint",
+  "resolve_socket",
+  "_send_remote",
+  "_send_remote_async",
+  "active_browser_targets",
+  "display_browser_name",
+  "remote_browser_targets",
+  "remote_browser_targets_async",
+  "remote_target_for_alias",
+  "send_command",
+  "send_command_async",
+]