feat: full windows #82

What

Windows today drag and collapse, stack by call order and have a fixed width. They need to be full windows before docking can build on them.

Layer bands

push_layer gives a child the parent's layer plus one, so sibling popups and windows land on the same layer and window z-order has nowhere to live. Replace that with layer bands defined as data: base content, docked content, floating windows in z-order, overlays (#84), popups and menus, modals (#100), tooltips, and the drag preview (#99). The context assigns each window a layer within its band from its z-order, and nested popups still stack above their parent within the popup band.

Needs #94 (stable ids) and #95 (window state and persistence). #96 supplies resize cursors.

Acceptance

• Layer bands as data, with routing and paint order following them, and tests for sibling popups and reordered windows
• Resize from every edge and corner, with min and max sizes, showing resize cursors
• Z order kept by the context: clicking a window brings it to front, independent of call order
• Close button, auto-size to content, and a scrollable body
• Window identity is its hashed id, so state survives reorderings of the calling code
• Position, size, open and collapsed state persist through the state store's save and load
• Windows can be pinned or locked, and declare flags (no title, no resize, no move, no background)