perf: Arc-cache paper_canvas_wires() to eliminate O(N) clone on navigation

paper_canvas_wires() now returns Arc<Vec<WireModel>> backed by
paper_canvas_cache (epoch-keyed). Pan/zoom frames hit the cache and
return a pointer copy; only geometry changes rebuild the wire list.
Completes the paper-canvas caching chain (L+M): sheet cache →
projected-per-viewport cache → full canvas Arc cache.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/paper_canvas.rs

@@ -100,7 +100,7 @@ impl<'a> canvas::Program<Message> for PaperCanvas<'a> {
         };
 
         // ── Wires (entity lines + inactive viewport projections) ──────────────
-        for wire in &self.scene.paper_canvas_wires() {
+        for wire in self.scene.paper_canvas_wires().iter() {
             let [r, g, b, a] = wire.color;
             let color = Color { r, g, b, a };