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/native/protocol.py

@@ -0,0 +1,30 @@
+"""Chrome Native Messaging stdio protocol helpers."""
+from __future__ import annotations
+
+import json
+import struct
+
+def read_exact_stream(stream, n: int) -> bytes | None:
+  buf = b""
+  while len(buf) < n:
+    chunk = stream.read(n - len(buf))
+    if not chunk:
+      return None
+    buf += chunk
+  return buf
+
+def read_native_message(stream) -> dict | None:
+  raw_len = read_exact_stream(stream, 4)
+  if raw_len is None:
+    return None
+  msg_len = struct.unpack("<I", raw_len)[0]
+  data = read_exact_stream(stream, msg_len)
+  if data is None:
+    return None
+  return json.loads(data.decode("utf-8"))
+
+def write_native_message(stream, msg: dict) -> None:
+  data = json.dumps(msg).encode("utf-8")
+  stream.write(struct.pack("<I", len(data)))
+  stream.write(data)
+  stream.flush()