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_framing.py

@@ -0,0 +1,59 @@
+import asyncio
+import struct
+
+import pytest
+
+from browser_cli.framing import (
+    async_recv_frame,
+    frame,
+    recv_frame,
+    send_frame,
+)
+from browser_cli.version_manager import MAX_MSG_BYTES
+
+class FakeRecv:
+    def __init__(self, chunks):
+        self.chunks = list(chunks)
+
+    def recv(self, n):
+        if not self.chunks:
+            return b""
+        chunk = self.chunks.pop(0)
+        if len(chunk) > n:
+            self.chunks.insert(0, chunk[n:])
+            return chunk[:n]
+        return chunk
+
+def test_frame_prefixes_payload_length():
+    assert frame(b"abc") == b"\x03\x00\x00\x00abc"
+
+def test_recv_frame_reads_chunked_socket_payload():
+    sock = FakeRecv([b"\x05", b"\x00\x00\x00he", b"llo"])
+    assert recv_frame(sock) == b"hello"
+
+def test_recv_frame_allow_eof_returns_none():
+    assert recv_frame(FakeRecv([]), allow_eof=True) is None
+
+def test_recv_frame_rejects_oversized_payload():
+    sock = FakeRecv([struct.pack("<I", MAX_MSG_BYTES + 1)])
+    with pytest.raises(ConnectionError, match="too large"):
+        recv_frame(sock, label="message")
+
+def test_send_frame_writes_prefixed_payload():
+    sent = []
+
+    class FakeSend:
+        def sendall(self, data):
+            sent.append(data)
+
+    send_frame(FakeSend(), b"ok")
+    assert sent == [b"\x02\x00\x00\x00ok"]
+
+def test_async_recv_frame_reads_payload():
+    async def run():
+        reader = asyncio.StreamReader()
+        reader.feed_data(frame(b"async"))
+        reader.feed_eof()
+        return await async_recv_frame(reader)
+
+    assert asyncio.run(run()) == b"async"