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/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