fix: implement CSI Ps b (REP) instead of silently dropping it
tmux's redraw logic leans on REP to compress long runs of a repeated
character (rule lines, padding/fill spans) into one write plus a
repeat count. Every unrecognized CSI final was a no-op, so those runs
silently collapsed to a single character — read on screen as text
stopping short with unexplained blank space (or stale content) after
it, worst the more such runs preceded a given point in a redraw.