PanelProps<'msg> Type
Immutable, compiler-checked authoring surface for a general-purpose panel. Feature 095 (E5): `Header` / `Footer` are the two CLOSED, typed chrome-region slots a consumer may fill with their own `Widget<'msg>` to re-skin the container's SHAPE (a custom header band / footer bar around the content). Each defaults `None`, which lowers to NO slot attribute (so an unfilled panel is byte-identical, FR-003). A filled `Header` lands ahead of `Children`, a filled `Footer` after — both inheriting E1–E4 + E2 retained identity by construction (FR-005).