ext-keybindings + ext-stage-dock: config-driven dock-toggle-visible (Super+d)

- **ext-stage-dock**: exports a `Service` interface with `toggle_visible()`.
  The extension inherits from it and registers via `provide_service` in
  `activate()`. The method slides the dock in/out (using the existing RCSS
  transition) regardless of slot count; showing an empty dock is valid.
- **ext-keybindings**: new `Action::dock_toggle_visible` action, mapped from
  `"dock-toggle-visible"` in `unbox.toml [[keybind]]`, dispatched to the
  stage-dock Service. Default binding: `Super+d`.
- **Manifest**: ext-keybindings now `depends_on {"xdg-shell", "stage-dock"}`.
- **Build**: subdir order swapped so ext-stage-dock builds before
  ext-keybindings; `ext_stage_dock_dep` is a link-time dep of the
  ext-keybindings library and transitively exposed via `ext_keybindings_dep`.
- **Tests**: glue tests install ext-stage-dock alongside ext-xdg-shell;
  policy test expects 6 default bindings. All 10/10 green on build +
  build-asan.

Configure in unbox.toml:
  [[keybind]]
  keys    = "Super+d"
  action  = "dock-toggle-visible"

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