Type optional readline adapter

Make the optional readline module's protocol boundary explicit. This keeps runtime behavior unchanged and allows strict mypy checks to agree across Unix and Windows.

Author: BenWestgate

src/codex32/_cli_input.py

@@ -6,7 +6,7 @@
 import os
 import sys
 from collections.abc import Callable, Iterator
-from typing import Protocol
+from typing import Protocol, cast
 
 from codex32.bip93 import (
     Secret,
@@ -46,7 +46,7 @@ def set_startup_hook(self, function: Callable[[], object] | None) -> None: ...
 except ImportError:
     _line_editor = None
 else:
-    _line_editor = _readline
+    _line_editor = cast(_LineEditor, _readline)
 
 
 class InputError(Exception):