A colour-picker drag rewrites doc fill on every mouse-move, so the scene
build cache (which compares the doc) rebuilt the whole layout — taffy + a
full SceneNode reshape — per drag frame, even though a solid colour change
touches no layout. Mirror the node-drag fast path: patch the anchor's
resolved scene fill / stroke via LayoutScene::set_node_fill /
set_node_stroke_color and invalidate the cache, skipping mark_dirty so the
patch survives the frame. Variable-mode, instance redirects, gradient-stop /
effect targets, and brand-new strokes fall back to the full rebuild.
Bumps vendor/jian to a86832d (the scene-patch methods). Native only; the
web host's live-sync push gate needs the same wiring once its spine settles.
e839c40770