SAFETY, LIGHTBURN: what the pause and the cancel mean, now that the button drives them

SAFETY: the chain de-energizes itself behind a pause without being asked -
the charge-pump feed ends with the run, so HV_ENABLE drops with the watchdog
about half a second in, and on the resume it is back ~216 ms before the first
step (pad measurements). A pause is deliberately not a cancel: the latch stays
unlocked and the window open, which is what lets the next press resume the
job, while emission still ends because the stream stops driving FIRE. The
disarm grace counts down through a hold, so a job left paused disarms itself,
and a lid or interlock open while paused takes the cancel path - nothing
resumes past an enclosure opening.

LIGHTBURN: the two things an operator needs before pausing a cut - the resume
restarts from where the deceleration ended and accelerates from a standstill,
which M3 shows as a deeper spot and M4 mostly hides, and a job left paused
disarms itself and asks for the button again. Stop leaves the head where it
stopped; returning to the job start belongs to the lid and interlock policy
alone.

docs/LIGHTBURN.md

@@ -113,6 +113,14 @@ restart the controller with the head re-parked.)
   queue + deceleration); Resume continues exactly. **The big button does
   the same**: one press while a job runs pauses it (LightBurn shows Hold),
   the next press resumes it — the factory's pause/resume, on the machine.
+  Two things to know before you pause a cut. **Resume where it stopped:**
+  there is no backtrack in GRBL mode, so the beam restarts from where the
+  deceleration ended and accelerates away from a standstill — at constant
+  power (`M3`) that leaves a deeper spot you can see, while `M4` scales power
+  with speed and mostly hides it. **Don't leave it paused:** the armed window
+  has an idle grace (`laser_disarm_s`, default 60 s) that counts down through
+  a hold, so a job left paused disarms itself and the resume asks for the
+  button again before it can fire.
 - **Opening the lid (or a Pro's interlock loop) during a job cancels it**,
   as the factory firmware does: the head parks with a controlled
   deceleration (the hardware cut the beam the instant the lid moved), the
@@ -128,9 +136,11 @@ restart the controller with the head re-parked.)
   material never leaves LightBurn waiting.
 - **Stop** = soft reset: motion aborts with a controlled deceleration
   and grblHAL raises an alarm with **position declared lost** (the
-  stream queue means up to ~40 mm of in-flight difference). Recovery:
-  unlock (`$X` in Console or LightBurn's prompt), jog the head clear,
-  and carry on in Current Position mode. Restart the controller with
+  stream queue means up to ~40 mm of in-flight difference). It leaves the
+  head where it stopped — the return to the job start belongs to the lid
+  and interlock policy alone, so Stop never moves the machine on its own.
+  Recovery: unlock (`$X` in Console or LightBurn's prompt), jog the head
+  clear, and carry on in Current Position mode. Restart the controller with
   the head re-parked if you want a clean absolute frame.
 - **Move tab**: jogging (set a sane speed, e.g. 6000 mm/min), Get
   Position, distance buttons.