perf(scene): skip re-tessellation on hover of selected entity

Phase 3.1 (partial). set_hover_highlight bumped the geometry epoch on
every hover-target change, forcing a full model re-tessellation. The
hover handle is folded into the tessellation highlight set as
selected ∪ {hover}, so hovering an already-selected entity contributes
nothing — the effective set and every resulting WireModel are identical.
Guard the bump on the effective highlight contribution actually changing;
hovering over or between selected entities is now free. The field is
still updated so hit-test / UI state stays current.

The full camera/selection-from-tessellation cache split (3.1 / 2.2) is
left open: highlight colour is baked into WireModel.color across several
tessellation sites, so decoupling it needs running-app verification.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/mod.rs

@@ -1367,8 +1367,20 @@ impl Scene {
     /// Set (or clear) the previewed entity that renders with the selection
     /// highlight without joining the real selection. Re-tessellates on change.
     pub fn set_hover_highlight(&mut self, handle: Option<Handle>) {
-        if self.hover_highlight != handle {
-            self.hover_highlight = handle;
+        if self.hover_highlight == handle {
+            return;
+        }
+        // The hover entity is folded into the tessellation highlight set
+        // (selected ∪ {hover}); see `wires_for_block_culled`. A hover handle
+        // that's already selected contributes nothing, so the effective set —
+        // and thus every WireModel — is identical. Only re-tessellate when the
+        // effective highlight contribution actually changes; hovering over (or
+        // between) already-selected entities then costs nothing. The field is
+        // still updated so hit-test / UI state stays current.
+        let contribution = |h: Option<Handle>| h.filter(|h| !self.selected.contains(h));
+        let changed = contribution(self.hover_highlight) != contribution(handle);
+        self.hover_highlight = handle;
+        if changed {
             self.bump_geometry();
         }
     }