Bake paint HUD layout into a prefab with separate channel panels

Split the single HUD panel into four independent per-channel sub-panels
plus a global brush-size panel, each a standalone RectTransform with its
own header (drag-handle ready). PaintHud now binds to a baked layout
prefab via recursive name lookup instead of building everything in code,
with a code-gen fallback. HsvColorPicker gains Rebind() to re-apply the
procedural gradient sprites and drag relays that a prefab cannot serialize.

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

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