[jsgrrchg]

Builds on the native Luau visuals from #900 to let users put their own art on the new-thread view. There are two new slots, both set in Settings → Appearance:

• Home visual: any visual script, drawn below the blank new-thread composer. It can be interactive (canvases, scenes, shaders, controls).
• Generative background: a ui.shader script as an alternative to the background image, drawn behind the composer. It moves but takes no input.

Summary

Home visual

• new_thread_visual (device-local) holds the script. Apply runs zeron_visual::check off the main thread and saves only if it passes; errors come back with their line. Clear removes it.
• VisualFrame::from_script runs a script already in hand with the same runtime as agent visuals.
• The shell places it from last frame's composer and column bounds. It fades with the Home selectors, leaves room for the bottom notices, clips at the terminal, and fades out at its bottom when it is taller than the room.
• ComposerInput gains a CodeInput key context, where Enter breaks the line, and a monospace option for code fields.

Generative background

• A background source setting, Image or Generative, sits next to the image. The image and the script are both kept, so switching loses neither. Choosing or shuffling an image switches back to Image.
• Apply checks that the script runs and returns a ui.shader without assets, and samples one frame's color for "Match wallpaper colors".
• Frames render at the background's size and paint through the same composer mask, fades, frost opacity, and dock dissolve as the image. Effects and framing don't apply.
• Cost controls:
  • frames render at half the display density, at most 1600 px on a side;
  • at most 30 fps, with one frame in flight;
  • frames render only while the background paints;
  • reduced motion and "Pause animations in background" hold a still frame.
• Without a GPU, the image shows instead.
• Readiness crossfades only between sources. A live source's frames replace each other in place, including mid-crossfade.
• The voice stage shows the same background.

Shared pieces

• ShaderPlayer is extracted from VisualFrame. It renders one shader's frames for agent visuals and for the background.
• ShaderSpec::fade_edges lets a host that masks the frame itself turn the edge fade off. Mobile and agent visuals keep it on.
• ScriptCard with a ScriptSpec is the settings card both slots share: code field, Apply, Clear, check errors.
• new-project-fixture takes ZERON_FIXTURE_HOME_VISUAL=<file.luau> and ZERON_FIXTURE_GENERATIVE=<file.luau>.

Test plan

• cargo test -p zeron-ui --lib -- settings:: composer:: visuals::: 357 passed.
• Focused run, -- new_thread_generative new_thread_background_effects settings::tests visuals: 89 passed. It includes the new tests:
  • the first shader is the background, and scripts without one are rejected with the error;
  • the background takes no pointer and spins on its own;
  • live frames replace in place, and only switching sources crossfades;
  • settings round-trip and stay omitted when unset.
• cargo test -p zeron-visual-gpu -- light_fades_out: a host-masked frame keeps its edges.
• cargo check for zeron-ui, zeron-visual-gpu (tests and examples), and zeron-mobile.
• Manual, new-project-fixture on Xvfb with lavapipe:
  • an animated shader and an animated canvas draw below the composer;
  • Apply shows a broken script's error and keeps the saved one;
  • a Pac-Man canvas game with an autopilot demo takes clicks to play;
  • a halftone "tokyo rain" shader renders as the background, masked and faded like the image, and animates.
• Not done: a real (non-fixture) app run, light themes, macOS/Windows, and an agent MCP tool for setting either script.

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