examples: ariadne_stream — the Phase-3 streaming holy-grail demo (stream as scene quad + stream_view window)

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Holy-grail step (d): one live video stream shown two ways from a single .ari — as the main scene (a type: stream node, a quad textured with the live frames) AND in a draggable/resizable ImGui window (a type: stream_view widget the same stream is routed to). The capstone of the Phase-3 streaming arc (#516–536).

Independent of unmerged work — everything it uses is on master (#532 camera, #536 stream_view, #533/#535 audio family + output, the substrate). The audio half (mic on/off/record/playback via the device source + audio_output) is a planned follow-up.

What it does

• ariadne_stream.cpp (host): opens one stream at the app root, registers the two sinks (register_stream_node_type + register_stream_view_widget), loads stream.ari, realizes the scene, renders — a navigable window or a one-frame offscreen PNG. Source is a hardware-free synthetic test pattern by default; --camera feeds a real SDL3 camera when present.
• stream.ari: declares the scene quad + a "Camera" window holding the stream_view widget, both bound to token streams.cam0.
• Sizes the stream pool for both sinks (expected_subscribers=3) — the pool is fixed-size and refuses rather than under-provisions, so a second sink on one token gets no frames unless budgeted. (Found this while verifying.)

Verification (headless EGL): ariadne_stream stream.ari --offscreen --png renders the live synthetic stream textured onto the scene quad — see the attached capture; the colorful gradient is the moving test pattern mapped through substrate → type: stream node → GPU texture. The stream_view window renders in an interactive session: cvcGL's offscreen path does not composite the ImGui overlay (ariadne_hello hides it in captures for the same reason), so the headless still shows the scene quad only. The widget's pull logic is covered hardware-free by cvcgl_stream_view, and its draw reuses the proven Kind::Image path.

Note: a backend-registered custom widget (stream_view) draws a cosmetic load-time "no registered handler" warning (the loader is backend-neutral); the widget works.