Version 1.1 added graphics buffer switching (CMD_SET_BUF_LB / CMD_SET_BUF_UB). The switching is synchronized with VBlank to avoid tearing — the buffer swap happens only during the vertical blanking interval (v_cnt >= 480).

Implementation:
- Added vblank output to vga_timing.v
- Added ST_WAIT_VBLANK state to cpu_ram.v
- Buffer swap is triggered by rising edge of vblank

This ensures the VGA display never shows a partially-drawn frame.