feat: a main menu bar takes its row from the free area #196

What

A menu bar across the top of the screen is the next item of the layout, but it does not take its row from the free area (context.free_area), which only docked containers reduce. An app with a main menu bar and a dock space beneath it has to clamp the dock space below the bar itself (octalide/ac's monitor does). Dear ImGui's main menu bar shrinks the viewport's work area for this reason.

Acceptance

• A main menu bar takes its row from the top of the free area, as a docked container would, so free_area read after it starts below the bar
• It behaves as begin_bar/end_bar otherwise: menus, shortcuts and keyboard navigation unchanged
• A test lays out a main bar and a dock space from free_area and checks they do not overlap