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_refactor_boundaries.py

@@ -18,7 +18,7 @@ class TestEndpoints:
 
         assert _normalize_endpoint("example.com:443") == "example.com"
         assert _normalize_endpoint("example.com:8765") == "example.com:8765"
-        assert _normalize_endpoint("192.168.1.10:443") == "192.168.1.10:443"
+        assert _normalize_endpoint("203.0.113.10:443") == "203.0.113.10:443"
 
     def test_resolve_connect_endpoint_defaults_domain_to_443(self):
         from browser_cli.endpoints import _resolve_connect_endpoint
@@ -69,7 +69,7 @@ class TestReExports:
     def test_client_still_exposes_moved_names(self):
         from browser_cli import client
 
-        # moved to endpoints / remote_transport / _errors but still reachable here
+        # moved to endpoints / remote.transport / errors but still reachable here
         for name in (
             "BrowserNotConnected",
             "_normalize_endpoint",
@@ -90,16 +90,16 @@ class TestReExports:
 
     def test_patching_client_send_remote_still_intercepts(self):
         # send_command resolves _send_remote as a browser_cli.client global, so
-        # patching there must still take effect after the move to remote_transport.
-        with patch("browser_cli.client._send_remote", return_value=None) as fake:
-            assert browser_cli.client._send_remote is fake
+        # patching there must still take effect after the move to remote.transport.
+        with patch("browser_cli.client.core._send_remote", return_value=None) as fake:
+            assert browser_cli.client.core._send_remote is fake
 
 # ── BrowserCLI mixin composition ─────────────────────────────────────────────
 
 class TestMixinComposition:
     def test_factory_builds_bound_tab(self):
         b = BrowserCLI()
-        tab = b._make_tab({"id": 7, "windowId": 1, "title": "t", "url": "u"})
+        tab = b.tab_from({"id": 7, "windowId": 1, "title": "t", "url": "u"})
         assert tab.id == 7
         assert tab._browser is b
 
@@ -111,7 +111,7 @@ class TestMixinComposition:
             display_name = "host:work"
             remote = "host:8765"
 
-        tab = b._make_tab_for({"id": 1, "windowId": 0}, _Target())
+        tab = b.tab_from_target({"id": 1, "windowId": 0}, _Target())
         assert tab.browser == "host:work"
         assert isinstance(tab._browser, BrowserCLI)
         assert tab._browser is not b