Root cause found via datasheet: PxIM0/PxIM1 has no dual-edge mode, only
falling/rising/low-level/high-level. (IM1=1,IM0=1) was actually
high-level interrupt, refiring continuously for the whole high-pulse
duration (confirmed by scope: no real HF noise, and edge_count=0 when
P3.6 grounded). Fix: start in rising-edge mode, software-toggle to the
opposite edge on every trigger (ping-pong) to emulate true dual-edge
triggering, feeding exact edge direction into the incremental decode
state machine (no glitch filter needed). Also switch P3.6 idle bias
from pull-up to pull-down, since floating-high under the old high-level
mode caused the interrupt-storm boot issue; removed the now-conflicting
pull-up restore in Wakeup_Restore().
