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.

scripts/bench/live_fire_drills.py

@@ -7,7 +7,7 @@ watch, an extinguisher, and the exhaust running. Every drill waits for
 the operator to press the physical arm button before the machine fires;
 nothing here defeats that gate.
 
-Usage: live_fire_drills.py <drill> [S] [F]   (S, F used by ircut)
+Usage: live_fire_drills.py <drill> [S] [F]   (S, F used by ircut, pthresh)
 
 Drills (pass a name):
   witness   Phase 5 A-1/A-2/A-5: a short vector mark at S400. Samples
@@ -33,6 +33,15 @@ Drills (pass a name):
             >= 3 times on representative material; the highest peak
             delta sizes cool_fire_ir_delta.
               ircut [S] [F]             e.g. ircut 1000 300
+  pthresh   Laser power-threshold ladder: one line per power level on
+            scrap, climbing from 2 % to 30 % of full, at constant power
+            (M3) so nothing scales the duty with velocity. The lowest
+            rung that leaves a mark is the tube's striking threshold,
+            and because $35 is a percent of full duty and the rungs are
+            percents of $30 with $31 = 0, that rung's percent IS the
+            $35 value. Requires $35 = 0 for the run: a floor already in
+            place lifts every rung and hides the threshold.
+              pthresh [Smax] [F]        e.g. pthresh 1000 300
   expstop   Armed kill on the EXPECTED-stop path: start a mark job,
             then mid-burn POST /controller/stop (the supervisor stops
             the controller: SIGTERM, reap, exit safing). PASS: emission
@@ -414,6 +423,71 @@ def drill_ircut(g):
     return samples
 
 
+# Power ladder for `pthresh`, in percent of full duty. The spacing is fine
+# at the bottom because that is where the tube stops striking.
+PTHRESH_PCT = (2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 20, 25, 30)
+PTHRESH_LEN = 25.0                      # mm of burn per rung
+PTHRESH_PITCH = 3.0                     # mm between rungs
+
+
+def drill_pthresh(g):
+    smax = int(sys.argv[2]) if len(sys.argv) > 2 else 1000
+    feed = int(sys.argv[3]) if len(sys.argv) > 3 else 300
+    levels = [(p, max(1, int(round(smax * p / 100.0)))) for p in PTHRESH_PCT]
+    print('=== laser power threshold ladder: %d rungs, F%d, %g mm each ==='
+          % (len(levels), feed, PTHRESH_LEN))
+    print('constant power (M3): the commanded duty is the tested duty.')
+    print('PRECONDITION: $35 must be 0 for this run. A floor already in')
+    print('place lifts every rung and the threshold cannot be read.')
+    print('rungs (drawn in order, alternating direction, +Y between):')
+    for i, (pct, s) in enumerate(levels):
+        print('  %2d: %2d%% -> S%d' % (i + 1, pct, s))
+    print('connect: %s' % prepare(g))
+    base = sample_forgectrl()
+    print('pre-fire: %s' % base)
+    arm_cue()
+    print('>>> This ladder reaches %d%% of full power - use scrap you are'
+          % PTHRESH_PCT[-1])
+    print('>>> willing to cut through.\n')
+    job = ['G91', 'G21', 'M3']
+    for i, (_pct, s) in enumerate(levels):
+        job.append('S%d' % s)
+        job.append('G1 X%g F%d' % (PTHRESH_LEN if i % 2 == 0 else -PTHRESH_LEN,
+                                   feed))
+        job.append('G0 Y%g' % PTHRESH_PITCH)
+    job += ['M5', 'G90', 'M2']
+    samples = run_and_sample(g, job, overall_timeout=600)
+    hv_vals = [s['hv'] for s in samples if s['hv'] is not None]
+    emis = [s['emission'] for s in samples if s['emission'] is not None]
+    print('\n--- results ---')
+    print('samples: %d  emission peak=%s' % (len(samples),
+                                             max(emis) if emis else '-'))
+    print('hv_current range: %s..%s' % (min(hv_vals) if hv_vals else '-',
+                                        max(hv_vals) if hv_vals else '-'))
+    if samples:
+        t0 = samples[0]['t']
+        print('hv_current trace (t s : raw) - the discharge current is the')
+        print('electrical witness of striking; it lifts off baseline at the')
+        print('same rung the material starts marking:')
+        line = []
+        for s in samples:
+            if s['hv'] is None:
+                continue
+            line.append('%5.1f:%s' % (s['t'] - t0, s['hv']))
+            if len(line) == 8:
+                print('  ' + '  '.join(line))
+                line = []
+        if line:
+            print('  ' + '  '.join(line))
+    print('\nRead the material: count rungs from the FIRST one drawn. The')
+    print('lowest rung that leaves any mark is the striking threshold; set')
+    print('$35 to that rung\'s percent (round up to the next rung for')
+    print('margin). Note the emission counter proves the safety chain')
+    print('asserted LASER_ON, not that the tube lased - only the mark and')
+    print('the discharge current say that.')
+    return samples
+
+
 def post_ctrl(action):
     # http.client preserves the header-name case exactly as given.
     import http.client
@@ -499,6 +573,7 @@ def main():
     drill = sys.argv[1] if len(sys.argv) > 1 else ''
     drills = {'witness': drill_witness, 'hold': drill_hold,
               'faultpos': drill_faultpos, 'ircut': drill_ircut,
+              'pthresh': drill_pthresh,
               'expstop': drill_expstop, 'ctrlstart': drill_ctrlstart}
     if drill not in drills:
         print(__doc__)