refactor: split compat into package, harden serve proxy (v0.9.3)

- compat.py → compat/ package: auth.py (auth-field normalizers),
  commands.py (command-format shims), __init__.py (re-exports)
- Add _auth_0_9_3 transformer: normalizes pubkey to lowercase before auth
  so clients < 0.9.3 sending uppercase hex are accepted
- adapt_auth() now called before auth check in serve.py; command extracted
  after adapt_auth so future transformers can rename commands safely
- serve.py: deduplicate _recv_exact (import from client), unify
  resp/resp_payload across Windows/Unix branches, require lowercase hex
  pubkey (re.fullmatch), reorganize imports, drop unused os import
- client.py: move payload/framed construction inside branches (remote path
  no longer serializes JSON it never uses); fix _is_valid_key_spec
  operator precedence; import MAX_MSG_BYTES from version_manager
- auth.py: narrow except clause (ValueError instead of bare Exception)
- Bump version 0.9.2 → 0.9.3

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

browser_cli/compat/__init__.py

@@ -0,0 +1,4 @@
+from browser_cli.compat.commands import adapt_request, adapt_response
+from browser_cli.compat.auth import adapt_auth
+
+__all__ = ["adapt_auth", "adapt_request", "adapt_response"]

browser_cli/compat/auth.py

@@ -0,0 +1,44 @@
+"""
+Auth-field normalizers — applied to the raw incoming message *before* the
+auth check runs.  Protocol fields (pubkey, sig, …) are still present here.
+
+Add one entry per breaking auth-field change:
+    ("X.Y.Z", transformer_fn)
+
+Entries must stay in ascending version order.
+"""
+from __future__ import annotations
+from typing import Callable
+from browser_cli.version_manager import parse_version
+
+
+# ── v0.9.3 ────────────────────────────────────────────────────────────────────
+
+def _auth_0_9_3(msg: dict) -> dict:
+    """pubkey validation tightened to lowercase hex; normalize for older clients."""
+    changed: dict = {}
+    pk = msg.get("pubkey")
+    if isinstance(pk, str) and pk:
+        changed["pubkey"] = pk.lower()
+    if msg.get("command") == "browser-cli.auth.trust":
+        args = msg.get("args") or {}
+        trust_pk = args.get("pubkey")
+        if isinstance(trust_pk, str) and trust_pk:
+            changed["args"] = {**args, "pubkey": trust_pk.lower()}
+    return {**msg, **changed} if changed else msg
+
+
+# ── registry ──────────────────────────────────────────────────────────────────
+
+_AUTH_COMPAT: list[tuple[str, Callable[[dict], dict]]] = [
+    ("0.9.3", _auth_0_9_3),
+]
+
+
+def adapt_auth(msg: dict, client_version: str) -> dict:
+    """Apply all auth normalizers needed to bring msg up to the current format."""
+    cv = parse_version(client_version)
+    for version, fn in _AUTH_COMPAT:
+        if cv < parse_version(version):
+            msg = fn(msg)
+    return msg

browser_cli/compat/commands.py

@@ -0,0 +1,43 @@
+"""
+Command-format shims — applied to clean_msg (protocol fields already stripped)
+before forwarding to the native host, and to responses before sending back.
+
+Add one entry per breaking command-format change:
+    ("X.Y.Z", request_fn, response_fn)
+
+- request_fn(msg: dict) -> dict   or None
+- response_fn(resp: bytes, command: str) -> bytes   or None
+
+Entries must stay in ascending version order.
+adapt_request walks forward (oldest first); adapt_response walks backward.
+
+Current baseline: 0.9.3 — no command-format shims needed yet.
+"""
+from __future__ import annotations
+from typing import Callable
+from browser_cli.version_manager import parse_version
+
+
+# ── registry ──────────────────────────────────────────────────────────────────
+
+_COMPAT: list[tuple[str, Callable[[dict], dict] | None, Callable[[bytes, str], bytes] | None]] = [
+    # ("1.0.0", _req_1_0_0, _resp_1_0_0),
+]
+
+
+def adapt_request(msg: dict, client_version: str) -> dict:
+    """Upgrade a client message to the current browser command format."""
+    cv = parse_version(client_version)
+    for version, req_fn, _ in _COMPAT:
+        if cv < parse_version(version) and req_fn is not None:
+            msg = req_fn(msg)
+    return msg
+
+
+def adapt_response(resp: bytes, command: str, client_version: str) -> bytes:
+    """Downgrade a native-host response to the format the client expects."""
+    cv = parse_version(client_version)
+    for version, _, resp_fn in reversed(_COMPAT):
+        if cv < parse_version(version) and resp_fn is not None:
+            resp = resp_fn(resp, command)
+    return resp