Creating or freeing a frame or region re-derives only what it touches (#915)

Any write that changed the layout graph's roster (a frame or region created, freed, re-owned or re-pointed) made the next resolve derive the whole graph: about 1.35 ms for one `CreateTexture` or `CreateFrame` on the stock UI, against 0.05 ms for a no-op.

A write now names the nodes it touched, and the resolve re-derives just those in the cached graph: a joining node is added with its edges, a leaving one is removed and its dependents seeded, a staying one has its edges and inputs re-synced. The full derivation stays the fallback. A region's owner and an art change are now layout inputs that name their region. Verify now also compares the graph itself.

A randomized test checks rects and graph against a scratch solve after every batch of creates, frees, re-owns and re-anchors. `CreateTexture` now costs 0.14 ms, `CreateFrame` 0.15 ms, a backdrop rebuild 0.22 ms.

Fixes #903

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -318,8 +318,11 @@ fn build(model: &mut Model, fh: FrameHandle, blocks: &[Block]) {
         }
     }
 
+    // Each new block joins the graph; the old ones' frees named theirs.
+    for &rh in &made {
+        model.touch_layout_region(rh);
+    }
     model.simple_html.entry(fh).or_default().blocks = made;
-    model.touch_layout();
 }
 
 /// A block's anchor (`SetPoint` `0x767c70`): block 0 to the frame, block N to the previous text