perf: share unchanged layers between undo states

Every undoable edit keeps the previous project for undo, so the comp it touches
is shared and Arc::make_mut deep-copies it: every layer and every keyframe. An
edit therefore costs time in proportion to the comp's size, and a script that
sets many keyframes in a big comp goes quadratic (a 1,600-layer build with
~100k sampled keys took ~45 minutes).

Comp::layers is now a Layers collection backed by Vec<Arc<Layer>>: copying the
stack copies pointers, and only the layer an edit changes is copied. The API
mirrors Vec<Layer>; serialized form is unchanged (a list of layers).
Comp::layer_mut and the iter_mut().filter/find call sites use find_mut /
iter_mut_where so they copy only the layers they actually change.

- prop.addKey in a 400-layer comp: 1.36 ms -> 0.16 ms per key (flat in comp size)
- the 1,600-layer script build: ~45 min -> 47 s, frames pixel-identical
- regression tests: an edit leaves the other layers shared with undo; layers
  round-trip as a plain list

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VkpGcKfWyCudfSsQNiWAaG

apps/effectcraft-cli/src/main.rs

@@ -1189,7 +1189,7 @@ fn adv3d_bench_comp(p: &mut effectcraft_engine::project::Project) -> Option<Item
         set(l, "cameraOptions/highlightGain", Value::Scalar(20.0));
         set(l, "cameraOptions/highlightThreshold", Value::Scalar(200.0));
     });
-    comp.layers = layers;
+    comp.layers = layers.into();
     Some(p.add_item("Advanced 3D (bench)", effectcraft_engine::color::Label::None, None, ItemKind::Comp(comp.into())))
 }