splits.rs had grown to 551 lines after adding the design's pane header.
Move the header renderer, the close glyph, the URL helpers, and the
compact-canvas placeholder into chrome::split_pane so splits.rs is back
to a 422-line action + render orchestrator and the new helpers stay
under their own 150-line file.
Behaviour preserved: the pane click still selects the tab, the close
glyph still selects+closes, and pane_host/path/secure still derive from
the same UrlText fields.
ZacharyZhang-NY
ab8a08a754