Constraints to re-apply when the source content has been replaced wholesale
(--live). The line index is rebuilt from scratch each time, so caps that
were originally honored at startup need to be reasserted.
The display-config subset shared by every pane, focused or not. Both the
startup --split pane (main::build_second_pane) and the runtime :vsplit
pane call this so they stay in lockstep. Format-specific predicates
(filter/grep/format/display) and the --tabs/--header spec parsers live
in the main binary and are NOT applied here: build_second_pane layers
them on after this call, while runtime panes show the plain file (documented
v1 limitation). Images are pinned to ASCII by force_cell_mode, which the
caller invokes.