fire_test.py phases A/B with the scope on the PSU LASER_ON pin, power
byte 0 throughout, HV unpowered, operator-executed unlock: latch LOCKED
severs FIRE entirely (pin flat + kernel laser_enable 0 under 40k
streamed fire bits); latch unlocked with the chain unarmed shows FIRE
live kernel-side while the PSU pin stays flat - the factory board
gates LASER_ON behind OK_2_FIRE exactly like the OpenGlow AND design.
interlock_circuit semantics pinned by the 13-to-7 transition: b0
LASER_ON monitor active low, b1 FIRE active high, b3 latch 1=locked.
The 1-tick FIRE drop measurement is deferred (SoC net inaccessible;
PSU pin needs an armed chain) - documented with both closure paths.
