sl0p(bar): a new-tab button, and margins that match the pane gap

First pass of the status bar overhaul, in the zjstatus fork plus the layout
that configures it.

margin_left / margin_right: the bar is a full-width pane while every pane above
it is inset by the pane gap, so its ends stuck out past every frame on screen.
The margins are applied by rendering into an inset viewport (inset_state),
which is the same thing click positions are translated by -- so what is drawn
and what is clickable come from one definition. That is the split button's
lesson applied before shipping rather than after.

{new_tab}: a clickable button that opens a tab and focuses it, with no name and
no cwd, i.e. exactly what the `n` binding in TAB mode does. Clicking a tab
already switches to it and double-clicking renames it; making one was the hole
in the strip. Rendering and hit-testing are the same measured string here, so
the button covers exactly the columns it draws.

Both keys are inert when unset, so an unmodified zjstatus config renders
identically.

packaging/checks/status-bar-live.py reads the bar off a real session and
asserts it spans exactly the columns the pane frames span, clicks the + and
expects one more tab, then clicks beside it and expects none. Confirmed by
setting the margins to 0 and watching the span assertion fail: (0, 99) against
the frames' (2, 97).

The check counts painted cells rather than ink, because a bar segment begins
with a padding space inside its coloured pill -- strip the colour and the left
edge reads one column off, which is half the margin being measured.

0 files changed, 0 insertions, 0 deletions