tab bar: a sidebar that fits the terminal, and gives up the edge last

`tab_bar_width` was a reservation: the configured columns came off the
screen whatever the screen was, and the only fallback was all the way back
to the top row. On a narrow terminal that is chrome displacing the thing it
is chrome for, and the jump to the top moves every target you were aiming
at.

It is a ceiling now. app_tab_bar_cols takes the app, because the answer
depends on the terminal and not only on the config: held to the columns
left after a pane's collapse width, and to a third of the screen, and
narrowed rather than dropped. TAB_BAR_FIT (14) is the narrowest width this
code will invent on its own -- a config that asks for less is taken at its
word -- and 0 means there is no room for a sidebar at all, which is the one
case that still falls back on top.

Height counts too, which it did not before: a sidebar spends a row per tab,
so a wide but short terminal runs out of list before it runs out of screen,
and a tab with no row is not clipped, it is unreachable. tab_bar_min_rows
is that floor. Not when the top strip could not seat them either -- moving
them would gain nothing, so the side you configured stands.

All of it is re-derived per frame, like the layout, so a resize moves the
bar and nothing remembers that it did.

test_tab_bar also stops depending on where the suite is run from. An
unnamed tab borrows the basename of its focused pane's cwd, so eight checks
read `tests` under `make test` (which cds there) and `slosh` from the
repository root, where they failed. They are about padding, alignment and
the index, not about that derivation, so they name the tab first -- by id,
since `set-name` takes 0 for the focused *pane* only. 147 checks, green
from either directory.

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