SelectedBuildingPanel has grown to 1200 lines by carrying two unrelated
selection categories. Introduce the field half as its own widget first,
so the cut-over is a separate, reviewable step.
The panel owns only its own selection state and widgets: it renders the
single-object stats panel (ship, station, debris) or the multi-object
count summary, subscribes to the tick/commands-applied refresh signals
and the debug-draw toggle, and hides itself while it has no selection.
Which category owns the side panel is not its decision - the parent
feeds it through setSelectedEntities/setSelectedDebris/clearSelection.
Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG
