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/auth.py

@@ -174,7 +174,7 @@ def verify(pub_hex: str, nonce: bytes, msg: dict, sig_hex: str) -> bool:
         message = nonce + hashlib.sha256(canonical_payload(msg)).digest()
         pub_key.verify(bytes.fromhex(sig_hex), message)
         return True
-    except (InvalidSignature, Exception):
+    except (InvalidSignature, ValueError):
         return False