Engine: adding a 3D model switches a Classic 3D comp to Advanced 3D (#264)

Imported GLB/glTF models were invisible in the Composition viewer while
text showed. Only the Advanced 3D renderer draws model and primitive
layers, and new compositions (and New Comp from Selection) are Classic
3D by default, but adding a model layer left the comp's renderer alone,
so the model drew nothing on every backend. It wasn't the reporter's
GPU: on this machine the viewer showed nothing for a GLB in a new comp
with the GPU compositor (Vulkan) and with Mercury Software Only, and
the model in both once the comp used Advanced 3D.

As After Effects documents ("Advanced 3D gets automatically selected
when you add a 3D model to the composition"), layer.newModel /
layer.addItem of a model and layer.newPrimitive now switch a Classic
3D comp to Advanced 3D inside the same edit (one undo step, Geometry
Options added to its text and shape layers like Composition Settings
does) and toast that the comp changed renderer. Switching a comp that
has model layers back to Classic 3D toasts why they disappear.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -100,7 +100,8 @@ time), primitive layers (Layer ▸ New ▸ Cube/Sphere/Plane/Torus/Cone/Cylinder
 Geometry Options), text and shape layers with Geometry Options ▸ Extrusion Depth (glyph and
 fill outlines flattened, triangulated with holes and extruded with Angular/Concave/Convex
 bevels, per character so per-character 3D carries over), and a textured card for every other
-3D layer. Shading is physically based in linear light (glTF metallic-roughness; Cook–Torrance
+3D layer. Classic 3D doesn't draw model or primitive layers, so adding one switches a Classic 3D
+comp to Advanced 3D in the same undo step, as in After Effects. Shading is physically based in linear light (glTF metallic-roughness; Cook–Torrance
 GGX specular, Smith–Schlick geometry, Schlick Fresnel, Lambert diffuse, after Karis 2013):
 parallel, spot and point lights with After Effects' falloffs, ambient lights, image-based
 light from an Environment light whose Source (or the comp's Environment Layer, Layer ▸