Lit meshes under a Camera2d: populate normals so directional/ambient lighting works

[obiot]

Summary

lit: true only does anything under a Camera3d. On the 2D-camera (accumulated) path the engine never writes Mesh#normals — the array is documented as "world-space normals for the current draw, recomputed from originalNormals along the Camera3d path" — so the lit batcher pushes zeros and the shader has no surface to shade against.

Until recently that rendered black (normalizing a zero vector gives NaN). That is now guarded: a lit mesh with no usable normals degrades to unlit on both backends, which is recognisably the model rather than a hole. This ticket is about making it actually light.

Why it is not just "turn it on"

1. Normals need a CPU transform on that path. The Camera3d path rotates them on the GPU (mat3(uModelMatrix) * aNormal). The accumulated path has no model matrix at draw time, so normals would have to be rotated per vertex on the CPU. That is acceptable on this path specifically — it already CPU-projects every vertex, so it fits its own cost model rather than adding a new penalty. It is precisely the work #1507's retained path exists to avoid, which is presumably why it was never wired up.

2. Positional lights cannot work as-is — only directional and ambient. Lighting is evaluated in world space: the fragment stage compares vWorldPos against each light's world position for point/spot falloff. Under a 2D camera the vertices are the projected output, so vWorldPos would carry screen-ish coordinates and a point light's range would attenuate against projected pixel distances. That is not merely approximate, it is meaningless — and worse, it would look plausible.

Proposal

• Rotate originalNormals by the mesh's model rotation into normals on the accumulated path, so directional + ambient shading works.
• Make point and spot lights explicitly not apply there — ignored with a one-shot warning naming the camera, rather than silently shading against a bogus position.
• Optionally, later: thread a genuine world-space position through as a separate varying to lift that restriction, at the cost of another per-vertex attribute on a path that is already CPU-bound. Worth measuring before committing to it.

Acceptance

• An OBJ or raw-geometry mesh with lit: true under a Camera2d shades from a Light3d directional light plus ambient, and matches the Camera3d result for the same orientation.
• A point/spot light warns once and does not contribute, instead of producing a wrong result.
• A mesh with no normals at all still renders unlit (the current guard) rather than black.

References

src/renderable/mesh.js (originalNormals / normals, _projectVertices), src/video/webgl/batchers/lit_mesh_batcher.js (_pushVertex reads mesh.normals), src/video/webgl/shaders/mesh-lit.frag and src/video/webgpu/shaders/mesh-lit.wgsl (the zero-normal guard). Related: #1572 (OBJ normals), #1507 (retained path).