Custom-create now pre-creates a scene material and opens its inline editor
in the build pane (no separate right-side PaintPanel, which is removed);
the brush + "Paint with" use a scene: ref so painting stores the ref and
edits propagate everywhere. Slot preview (shared + item) resolves scene
refs so hover shows the real material.
Material properties editor uses the shared SliderControl for roughness/
metalness/opacity; row action buttons use Tooltip instead of title; the
color input renders as a clean filled swatch.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
