headless ant/term ↗

vt: implement background color erase (bce) for newline and erase ops TERM=xterm-256color declares bce in terminfo, so clients like tmux and htop trust that erasing a cell (EL, ED, ECH, ICH/DCH, IL/DL, and the blank rows scrolling exposes) fills it with the currently active SGR background rather than the default — that's why htop's row highlight stopped short of the right edge instead of reaching it. Adds a shared blank_cell(g) helper and wires it through clear_row, erase_line, erase_chars, insert_chars, and delete_chars (which transitively covers scrolling and insert/delete-line via clear_row). Also extends newline/NEL/autowrap to pick up an active background for cells past where the line's writer stopped. Fixes an ordering bug this surfaced in reset_grid (RIS): it cleared the screen before resetting cur_bg, so a reset right after setting a background would wipe to the stale color instead of the true default.