Introduce `HotkeySource` trait in `patina-boot`. Ship `KeyboardHotkeySource` (wraps SimpleTextInput/Ex scancode lookup) and `MockHotkeySource` (canned return for tests). Platform-button implementation lands separately (sibling sub-issue). Part of #50.
Introduce
HotkeySourcetrait inpatina-boot. ShipKeyboardHotkeySource(wraps SimpleTextInput/Ex scancode lookup) andMockHotkeySource(canned return for tests). Platform-button implementation lands separately (sibling sub-issue).Part of #50.