sidebar: align the numbers by default

The knob landed defaulting to "prefix" on the grounds that it is what the
strip has always drawn -- but the strip is not what this is about. A sidebar
row is wider than the name on it, and `1:api` spends the first two or three
cells of every row (three or four once you have ten tabs) on a number nobody
reads until they reach for C-a 2, pushing the names in by a distance that
depends on how many tabs are open. Right-aligned, the names start where the
statuses under them start and the numbers make a column of their own.

`tab_bar_index "prefix"` is still there and unchanged, and the top strip is
unaffected either way -- it ignores the setting, having no far end to align
against.

The tests that used `1:` as the anchor for "where a tab's name starts" now
anchor on the name, which is what they were about; two new ones cover the
default and the prefix form explicitly, so neither mode is tested only by
being the default.

0 files changed, 0 insertions, 0 deletions