Single ZephyrUSBCDC module owns the usbd context, 1200-baud DFU
detection, and DTR transitions for both roles. The boot banner
now blocks on a k_event signalled by the usbd_msg_callback when
DTR transitions high — host attached → wakes immediately; no host
→ bounded timeout (2 s repeater, 1 s companion). Replaces the
fixed k_sleep delays in both mains.
Deletes the companion's 10 s DTR-polling work — line state changes
arrive as events now, same callback handles disconnect (resets V3
parser, flips active_iface) and DFU touch (reboots to bootloader).
Side effect: prod companion no longer enumerates a phantom CDC ACM
port (CONFIG_LOG=n skips the whole stack instead of auto-initing
an unused device).
