feat(console): add core framework - keybindings, navigation, screen base

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>

console/core/screens.py

@@ -0,0 +1,46 @@
+"""
+Base screen class for the console UI.
+
+Every screen in the application inherits from :class:`Screen` and overrides
+:meth:`on_enter`, :meth:`on_key`, and :meth:`render`.
+"""
+
+
+class Screen:
+    """Abstract base for all console screens.
+
+    Subclasses must override the three lifecycle methods to provide real
+    behaviour.  The base implementations are intentional no-ops so that
+    simple screens (e.g. a static splash page) need not implement every
+    method.
+
+    Attributes:
+        name:  Machine-readable identifier used by :class:`Navigation`.
+        title: Human-readable heading displayed at the top of the screen.
+    """
+
+    def __init__(self, name: str, title: str):
+        self.name = name
+        self.title = title
+
+    def on_enter(self, context, db, renderer) -> None:
+        """Called once when this screen becomes the active screen.
+
+        Use this hook to load data, reset scroll positions, or set up
+        key bindings specific to the screen.
+        """
+
+    def on_key(self, key: int, context, db, renderer, nav):
+        """Handle a single keypress.
+
+        Returns a navigation instruction (e.g. a dict or tuple) when the
+        screen wants to push/pop, or ``None`` to stay on the current
+        screen.
+        """
+        return None
+
+    def render(self, context, db, renderer) -> None:
+        """Draw the screen contents using *renderer*.
+
+        Called after every keypress and on initial display.
+        """