feat: VKILL line kill, c_iflag ICRNL/IGNCR/INLCR, TCSETSW/TCSETSF

feat: VKILL line kill, c_iflag ICRNL/IGNCR/INLCR, TCSETSW/TCSETSF

- Add VKILL (Ctrl-U) handling in canonical mode: erases entire
  line buffer with visual backspace feedback
- Add c_iflag bits: ICRNL (default on), IGNCR, INLCR
- Add tty_iflag state variable, default ICRNL enabled
- c_iflag input translation runs before signal/canonical processing
- Replace hardcoded CR→NL conversion with c_iflag-based translation
- TCGETS now returns c_iflag; TCSETS now applies c_iflag mask
- Add TCSETSW (0x5403) and TCSETSF (0x5404) ioctl commands
  (treated same as TCSETS — no output queue to drain/flush)

20/20 smoke tests pass.