Add the three core modules that all screens depend on:
- keybindings.py: Key constants (curses codes) and KeyBindings registry
- navigation.py: Stack-based screen navigation with breadcrumbs
- screens.py: Screen base class with on_enter/on_key/render lifecycle
Includes 31 tests covering all public APIs.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
