Cube node shape renders black canvas via C API (CommandScreen doesn't implement DrawCube/FinishRender3D)

[willwade]

Problem

When LP_GEOMETRY is set to Cube (Options::CUBE), the canvas goes completely black — no nodes are visible. The app doesn't crash; it just renders only the background clear-screen and nothing else.

Root cause

CDasherViewSquare in Cube mode calls Screen()->DrawCube(...) for each node and Screen()->FinishRender3D(...) to composite the 3D layers. These are virtual methods on CDasherScreen (DasherScreen.h:98-103).

The C API's CommandScreen (in CAPI.cpp) — the screen adapter that translates DasherCore rendering into the flat int[] command buffer consumed by all frontends — never overrides these methods. They fall through to the empty base implementation, so zero draw commands are emitted for cube nodes.

Only the clear-screen command (opcode 0) fires — hence the black canvas.

The flat rectangle rendering (default node shape) works fine because it uses DasherRectangle() / DasherPolyline(), which CommandScreen does translate into the int[] buffer.

Suggested fix

Option A (preferred): Implement DrawCube + FinishRender3D in CommandScreen — translate cube quads into the flat int[] command format. The cubes are shaded rectangles with depth-based colour; they don't need actual 3D. This could emit opcode 4 (filled rectangle) for each cube face with the depth-shaded colour.

Option B: Have CDasherViewSquare detect that the screen doesn't support 3D (e.g., a Supports3D() capability check) and fall back to flat rectangle rendering. Simpler but loses the cube visual.

Context

• Reported via TestFlight (Dasher-Apple build 75+, iOS 27).
• Same class of problem as the Circle geometry crashes (fix(view): circle shape SIGABRT + CircleTo stack overflow #46) — a rendering mode the C-API path was never tested with.
• All frontends using dasher.h are affected (Apple, Windows, GTK, Android, Web).