Every FETCHed proactive command must be answered, otherwise the card is
left in an unfinished session and stops issuing commands (e.g. it will
not deliver a PoR for SEND SM). The menu handlers now share
_menu_send_response, and a server-side watchdog (_arm_menu_timeout /
_menu_timeout_fire, --menu-timeout, default 60s, 0 disables) sends the
timeout result (0x12) when the user never answers. The timer is armed
while a command is pending and cancelled on any response, equip, rescue
and card disconnect.
Also fixes docs/api.md, which had back (0x11) and timeout (0x12) codes
swapped. Tests for arm/cancel/clamping and the flat timeout TR. SW cache
v89 -> v90.
