#8740 moved the workspace header into every split pane so each pane could
pick full or compact chrome by its own width. Every pane then repeated the
title, project row, workspace menu and Explorer toggle, and narrow panes
swapped their tab rail for the tab counter.
The tab counter belongs to the single-pane layout. With two or more visible
panes, the split container now renders the header once above them (full or
compact by column width, without a counter) and every pane keeps its tab
rail. A single visible pane, including focus mode, keeps the merged compact
header with the counter when narrow.
