the layout graph is maintained, and the shine draws for real

- the interface's layout graph is maintained as anchors change instead of rediscovered from scratch: 1.48 ms per mutated frame becomes 34 µs, and end to end a running cast now costs +0.14 ms where it cost +4.33
- the autocast shine draws its authored population — 1200 particles through the star's own projection, spinning as the model says
- the shine is clipped to its own widget: the model frame installs its rect as the viewport, so nothing it draws leaves the button

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/benilla-ui/src/script/simplehtml/mod.rs

@@ -414,6 +414,13 @@ fn free_block(model: &mut Model, rh: RegionHandle) {
         model.id_to_region.remove(&id);
     }
     model.arena.destroy_region(rh);
+    // A death is the archetypal STRUCTURAL change — it takes a node out of the layout roster and
+    // its reverse edges with it, which is exactly what a per-node ledger cannot describe
+    // (decision 1388). `set_text`, this function's only caller, already ends in the same
+    // conservative touch, so this is redundant today; it is here because the redundancy is the
+    // point. A destruction site that does not invalidate the graph it destroys part of is a
+    // stale rect waiting for a second caller.
+    model.touch_layout();
 }
 
 // ─────────────────────────────────────────────────────────────────────────────────────────────