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/keybindings.py

@@ -0,0 +1,87 @@
+"""
+Key constants and key-binding registry for the console UI.
+
+Key provides named constants matching curses key codes so that screens
+and renderers never need to import curses directly.
+
+KeyBindings maps key codes to callable actions and tracks the footer
+labels displayed at the bottom of the screen.
+"""
+
+import curses
+
+
+class Key:
+    """Key constants matching curses key codes."""
+
+    ESCAPE = 27
+    ENTER = 10
+    TAB = 9
+    BACKSPACE = 127
+
+    UP = curses.KEY_UP
+    DOWN = curses.KEY_DOWN
+    LEFT = curses.KEY_LEFT
+    RIGHT = curses.KEY_RIGHT
+
+    PGUP = curses.KEY_PPAGE
+    PGDN = curses.KEY_NPAGE
+    HOME = curses.KEY_HOME
+    END = curses.KEY_END
+
+    F1 = curses.KEY_F1
+    F2 = curses.KEY_F2
+    F3 = curses.KEY_F3
+    F4 = curses.KEY_F4
+    F5 = curses.KEY_F5
+    F6 = curses.KEY_F6
+    F7 = curses.KEY_F7
+    F8 = curses.KEY_F8
+    F9 = curses.KEY_F9
+    F10 = curses.KEY_F10
+
+
+class KeyBindings:
+    """Registry that maps key codes to callable actions.
+
+    Usage::
+
+        kb = KeyBindings()
+        kb.bind(Key.ENTER, lambda: do_something())
+        handled = kb.handle(Key.ENTER)  # True, callback was invoked
+    """
+
+    def __init__(self):
+        self._bindings: dict[int, callable] = {}
+        self._footer_labels: list[tuple[str, str]] = []
+
+    def bind(self, key: int, action: callable) -> None:
+        """Register *action* as the callback for *key*.
+
+        If *key* already has a binding it is replaced.
+        """
+        self._bindings[key] = action
+
+    def handle(self, key: int) -> bool:
+        """Look up *key* and invoke its callback if one exists.
+
+        Returns ``True`` if a callback was found and executed,
+        ``False`` otherwise.
+        """
+        action = self._bindings.get(key)
+        if action is not None:
+            action()
+            return True
+        return False
+
+    def set_footer(self, labels: list[tuple[str, str]]) -> None:
+        """Set the footer bar labels.
+
+        *labels* is a list of ``(key_label, description)`` tuples, e.g.
+        ``[("F1", "Help"), ("F10", "Quit")]``.
+        """
+        self._footer_labels = list(labels)
+
+    def get_footer_labels(self) -> list[tuple[str, str]]:
+        """Return the current footer labels list."""
+        return list(self._footer_labels)