refactor: modularize auth transport and markdown

- Split auth into focused package modules for agent keys, file keys,
  signing, and post-quantum transport helpers while keeping the public
  browser_cli.auth import surface intact.
- Move transport encoding internals into a package with separate codec and
  binary-hoisting helpers, preserving browser_cli.transport compatibility.
- Extract remote TCP auth/socket helpers and serve challenge setup out of the
  runtime paths to make connection handling easier to reason about.
- Move the extension markdown extractor into a dedicated content/markdown
  folder with separate root selection, code normalization, renderer, and utils.
- Centralize CLI Rich rendering helpers for tab/window tree and table output,
  and add rendering tests for the shared builders.
- Remove local typing ignores in SDK/decorator/script plumbing and bump the
  package and extension version to 0.15.3.

browser_cli/transport/codecs.py

@@ -0,0 +1,84 @@
+"""Serialization/compression primitives for TCP response payloads."""
+from __future__ import annotations
+
+import gzip
+import zlib
+
+from browser_cli.constants import COMP_GZIP, COMP_NONE, COMP_ZLIB, COMP_ZSTD, SER_JSON, SER_MSGPACK
+
+try:  # optional: better ratio + speed than zlib/gzip
+  import zstandard as _zstd
+except Exception:  # pragma: no cover - depends on optional extra
+  _zstd = None
+
+try:  # optional: alternate serialization + raw binary for screenshots
+  import msgpack as _msgpack
+except Exception:  # pragma: no cover - depends on optional extra
+  _msgpack = None
+
+SERIALIZATION_NAME = {SER_JSON: "json", SER_MSGPACK: "msgpack"}
+SERIALIZATION_ID = {value: key for key, value in SERIALIZATION_NAME.items()}
+COMPRESSION_NAME = {COMP_NONE: "none", COMP_ZLIB: "zlib", COMP_GZIP: "gzip", COMP_ZSTD: "zstd"}
+COMPRESSION_ID = {value: key for key, value in COMPRESSION_NAME.items()}
+JSON_FIRST_BYTES = frozenset(b"{[")
+
+def msgpack_available() -> bool:
+  return _msgpack is not None
+
+def zstd_available() -> bool:
+  return _zstd is not None
+
+def supported_serialization() -> list[str]:
+  """Serializations this build can produce/consume, best first."""
+  return (["msgpack"] if _msgpack is not None else []) + ["json"]
+
+def supported_compression() -> list[str]:
+  """Compression codecs this build can produce/consume, best first."""
+  return (["zstd"] if _zstd is not None else []) + ["gzip", "zlib"]
+
+def client_accept_encoding() -> dict:
+  """What the local client advertises it can decode (sent with each request)."""
+  return {"ser": supported_serialization(), "comp": supported_compression()}
+
+def compress_payload(comp_id: int, data: bytes) -> bytes:
+  if comp_id == COMP_NONE:
+    return data
+  if comp_id == COMP_ZLIB:
+    return zlib.compress(data, 6)
+  if comp_id == COMP_GZIP:
+    return gzip.compress(data, compresslevel=6)
+  if comp_id == COMP_ZSTD:
+    if _zstd is None:
+      raise ValueError("zstd compression requested but zstandard is not installed")
+    return _zstd.ZstdCompressor(level=10).compress(data)
+  raise ValueError(f"unknown compression id {comp_id}")
+
+def decompress_payload(comp_id: int, data: bytes) -> bytes:
+  if comp_id == COMP_NONE:
+    return data
+  if comp_id == COMP_ZLIB:
+    return zlib.decompress(data)
+  if comp_id == COMP_GZIP:
+    return gzip.decompress(data)
+  if comp_id == COMP_ZSTD:
+    if _zstd is None:
+      raise ValueError("zstd payload received but zstandard is not installed")
+    return _zstd.ZstdDecompressor().decompress(data)
+  raise ValueError(f"unknown compression id {comp_id}")
+
+def choose_codec(accept: dict | None) -> tuple[int, int]:
+  """Pick (serialization_id, compression_id) the peer accepts, server preference first."""
+  accept = accept if isinstance(accept, dict) else {}
+  accept_ser = accept.get("ser") or ["json"]
+  accept_comp = accept.get("comp") or []
+
+  serialization = SER_JSON
+  if _msgpack is not None and "msgpack" in accept_ser:
+    serialization = SER_MSGPACK
+
+  compression = COMP_NONE
+  for name in supported_compression():  # server preference: zstd > gzip > zlib
+    if name in accept_comp:
+      compression = COMPRESSION_ID[name]
+      break
+  return serialization, compression