feat: docking #83

What

Dear ImGui style docking: windows dock into a dock space, split it, stack as tabs, and float out again, with the layout owned by the context and persisted.

Needs #82 (windows and layer bands), #99 (drag and drop carries the dragged window), #105 (tab bars for tab stacks) and #95 (the dock tree lives in and persists through the state store). Holding a modifier reads the held modifiers from #96.

Acceptance

• A dock space fills a rect (the whole surface or a region) and holds a tree of splits and tab stacks
• Dragging a window's title shows drop targets (center for tabs, four edges for splits, outer edges for the whole space), with a preview of the result
• Splitters between docked nodes resize them, with min sizes respected
• Tab stacks reorder by drag, close, and undock by dragging a tab out
• A dock layout can be built in code (a default layout) and saved and restored with the window state
• Docked windows keep their own scroll and state, and a docked window's content code is identical to a floating one's
• The existing begin_dock side panel becomes a dock node, so apps with a single side panel stay a one-call setup
• Holding a modifier while dragging suppresses docking