Design a panel's surface in the app, instead of by hand in the XML

[stoatworks-labs]

Surfaces have existed since panels stopped being flat, but the only way to shape one was to type control points into the stage file. The inspector now picks the kind — Flat, Arc or Lattice — and the Previz view drags a lattice's control points about.

What is in it

• Inspector: a surface kind picker; arc sweep with radius/chord/depth reported beside it; lattice columns/rows, a per-point X/Y/Z editor, Reset and Flatten.
• Previz: the selected panel's surface drawn as a wireframe, its control points as handles you can click and drag. Dragging anywhere else still orbits.
• Core: Surface::bake_lattice / arc_metrics / set_point, Panel::local_of / stage_of, Camera::project / ray / forward, and geom::Ray.

Three decisions worth keeping

• Conversions sample the old shape rather than rebuilding it from parameters, so switching an arc to a lattice mid-edit does not move the picture, and resizing the grid keeps the shape. Re-baking a lattice at its own size is the identity — the spinners re-bake on every change.
• Handles are picked from press_origin, not from where the pointer is when egui decides a press has become a drag. By that frame the pointer has left the handle, and hit-testing the live position picks nothing at all.
• A handle drags in the plane through it facing the camera, so depth is the one thing a pull cannot change.

Two things found on the way

• The viewport was rendered at the window's size and squashed into the rect that shows it — previz at the wrong aspect, and an emulation zoom that did not mean what it said. Had to go before handles could be dragged.
• The widgets are clickable after all, headlessly: egui::Context::run_ui takes a RawInput, so pointer events drive the real widget code with no window, no GPU and no NDI. The whole pick-drag-orbit path is tested that way, and it caught the press_origin bug before the app was ever run.

153 tests pass. Verified end to end as well: a real Arena 7.27 import, given an arc and a pulled lattice, renders through unmapper render --previz.

The formatting churn in the touched files is rustfmt catching up on lines that were already drifting.

🤖 Generated with Claude Code