Cover step timing under CPU contention; record the laser power model

Bump the grblHAL pin to the real-time producer change.

Add motion.step-timing-under-load: the catalog had nothing that
exercised step generation while userspace competed for the single core,
which is exactly the gap that let the condition go unnoticed - the ring
never runs dry, so cnc/underruns reads 0 through it. The test asserts
the producer and the shipper both hold SCHED_FIFO, then drives
2000 mm/min round trips against a deliberate nice-5 CPU hog and requires
the controller to report no clamped events.

Add the pthresh live-fire drill: a constant-power ladder from 2 % to
30 % of full on scrap. Because $35 is a percent of full duty and the
rungs are percents of $30 with $31 = 0, the lowest rung that marks reads
directly as the $35 value. It needs $35 = 0 for the run, or the floor
lifts every rung and hides the threshold.

Record both open items in BRINGUP. The laser one carries the finding
that the factory never uses duty as a power control - all five firing
jobs in the captured pulse files pin the power byte at 127 and modulate
dose by dithering the FIRE bit at 6.5-18.8 % density - so the captures
cannot supply a $35 default, and the duty to optical-power transfer
function of this supply has never been measured.

meta-forgefirm/recipes-forgefirm/grblhal-glowforge/grblhal-glowforge-pin.inc

@@ -2,5 +2,5 @@
 # changes; keep only SRCREV and PV here - the image manifest leaves
 # *-pin.inc out of the layer content hash because the component entry
 # already identifies the pinned source (forgefirm-image-manifest.bbclass).
-SRCREV = "0c192659f8c1c9ecf36a824c08bdcecd5085666a"
+SRCREV = "026c169c6ee028907a0235d4e3bbf5cf9d91e2a8"
 PV = "0.1.0"