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

crates/engine/src/render_queue.rs

@@ -676,7 +676,7 @@ impl Session {
                         continue;
                     }
                     if let Some(c) = proj.comp_mut(cid) {
-                        for l in c.layers.iter_mut().filter(|l| matches!(l.source, LayerSource::Comp { item } if item == comp)) {
+                        for l in c.layers.iter_mut_where(|l| matches!(l.source, LayerSource::Comp { item } if item == comp)) {
                             l.source = LayerSource::Footage { item: fid };
                         }
                     }