Nightly (5184cb2b5d46ad8e1e132083e309333a6e0b7508)

ariadne examples: declare requires: in every shipped document (#898)

* ariadne examples: declare requires: in every shipped document

Each document under examples/ariadne/ now lists what it needs from the host. Scene documents
require gl.scene, so ari (which has no scene adapter) refuses them with one error instead of
dropping the scene: block with a warning. Documents that call stream, room, nav or quit verbs
list those, and terminal_images marks raster.bitmap optional because half-block cells still
show the picture.

AriadneComponents.ShippedExamplesDeclareWhatTheyNeed loads every .ari under examples/ariadne and
checks that each declares requires:, and that any document with a scene: requires gl.scene.
docs/ARIADNE.md section 5.7 notes the convention.

Not built or run on the authoring machine (no MSVC toolchain for the existing build dir).

* ariadne: a terminal backend's widget is a known type, so requires: finds it

arigl --terminal registers stream_view through FtxuiBackend::register_widget,
which only stored the draw fn. ImGuiBackend::register_widget also calls
register_known_widget_type, which is what has_widget_type() (and so
requires: and (has? ...)) reads. So conference_media.ari's
requires: [stream_view] failed the load under --terminal
(arigl_terminal_stream_view, every package job), though the widget draws
there. The terminal backend now marks the type known too.