`InteractivePrinter::print_snip` computes the snip separator width as
`term_width - panel_count - snip_left_count - title_count`. At `--terminal-width 1`
the panel is disabled and `title_count == 2`, so the subtraction underflows
`usize` (release has no overflow-checks, so it wraps to ~usize::MAX); `str::repeat`
then aborts with "capacity overflow" whenever a snip separator is emitted (two or
more disjoint line ranges, or a diff gap) — which the default style does.
Use `saturating_sub` for the repeat counts so they clamp to 0 instead of
underflowing. Added an integration test.
