instance3d: a dedicated 3D Layer Instance effect, and the copy is cached

CC Composite stays After Effects-pure: its Layer parameter is gone, so
parameter indices match AE again (scripts and expressions that address
parameters by index). The model reference lives in its own effect,
ec.3d.instance3d (3D Channel, CPU, one layer parameter), which replaces
the layer's pixels with the referenced layer's render — alpha included.

A referenced model's solo render is now hashed into the owner's cache key
instead of declining the entry (Bflat, review of #2410): the layer, its
animated values and transform, the mesh item, the comp camera and lights,
draft 3D, the renderer setting and the in/out gate. Covered by a render
test: the copy is cached, served on the second render, and re-rendered
when the camera pans or the model leaves its in/out range.

crates/engine/src/tests/instance3d.rs

@@ -1,6 +1,6 @@
 //! Create 3D Layer Instance (Layer ▸ Create): a comp-sized solid at the top of the layer stack
-//! with CC Composite referencing the selected 3D model layer — a 2D copy of its render that 2D
-//! effects can be applied to (behaviour, pixels, undo/redo, serde).
+//! with a 3D Layer Instance effect referencing the selected 3D model layer — a 2D copy of its
+//! render that 2D effects can be applied to (behaviour, pixels, undo/redo, serde).
 
 use std::sync::Arc;
 
@@ -97,9 +97,9 @@ fn instance3d_creates_a_2d_copy_of_the_model() {
     let l = layer_in(&s, comp, solid);
     assert_eq!(l.name, "Triangle Instance");
     assert_eq!(s.active_comp().unwrap().layers[0].id.0, solid, "at the top of the layer stack");
-    // CC Composite referencing the 3D layer.
+    // A 3D Layer Instance effect referencing the 3D layer.
     let g = l.effects().unwrap().groups().next().unwrap().clone();
-    assert!(matches!(&g.kind, GroupKind::Effect { effect } if effect == "ec.channel.cccomposite"), "{:?}", g.kind);
+    assert!(matches!(&g.kind, GroupKind::Effect { effect } if effect == "ec.3d.instance3d"), "{:?}", g.kind);
     assert_eq!(g.get("layer").unwrap().value, KV::Layer(Some(model)));
     // The model is switched off, the copy selected.
     assert!(!layer_in(&s, comp, model).switches.video, "the referenced 3D layer is off, as After Effects does");