Cover the minimum pulse width; record what the density ladders measured

Rule 15 in the stream harness holds both halves of the minimum
(grblHAL-glowforge f7e8c17, pinned here): no emitted burst falls below
laser_pulse_min_ticks, excepting one clipped by fire going off mid-burst,
and the levels too faint to fill a window still render their exact
average density. Checked against a run at minimum 1 so it cannot pass
vacuously - level 2 goes from 444 bursts of one tick to 147 of three at
the same density, and levels already above the minimum are unchanged.

Four bench ladders settle the base period at 20. The same six rungs
marked in all of them, and the matched pairs across periods separate the
variables: at identical pulse length, halving the density killed the
mark; at identical density, varying the pulse 3x changed nothing. Feed
does not move it either - 10 percent at F100 carries 44 percent more
energy per mm than 20 percent at F300, which marks, and still left
nothing. The low-end marking limit is average power, not dose per length
and not pulse length.

The F100 trace separates two failures that look alike on the material:
seven current segments for eight rungs, anchored by a flat saturated
final segment that can only be full density, put 5 percent at no
discharge at all and 10 percent at a full 15 seconds of current with no
mark. Only the first is ours, and the minimum pulse is the answer to it.

Also recorded: the factory's Precision Power 1 runs a 19.53 percent FIRE
duty cycle at full PWM duty, and its 1-100 scale maps onto density
18.9-79.5 percent, so its 1 percent is the bottom of the useful band
rather than 1 percent of the range. Under the density model $35 and $36
are that same control - a density floor and ceiling - which makes the
user-facing scale a settings choice rather than new code.

docs/BRINGUP.md

@@ -212,9 +212,16 @@ finer than one tick per period average out. The model is selected per arm
 and reported (`laser armed (density)`). Density is what the tube's dead band
 below its lasing threshold requires: every pulse it emits is full-power, so
 no commanded level lands in the band, and a level change inside a run costs
-no stream byte at all. It wants `$35` = 0 - the floor exists only to keep an
-analog duty out of the band, and under density it just clamps the light end
-of the range; the arm warns when a floor is set. Structurally the model is a
+no stream byte at all. `laser_pulse_min_ticks` (default 3 = 106 us) is the
+shortest pulse it will emit: below it a period is skipped and its debt
+carried, so a faint level arrives as fewer full-width pulses instead of
+stubs the supply cannot strike - measured on the bench, a 36 us stub draws
+no discharge at all, and the factory never emits below one of its 100 us
+ticks. The debt is conserved, so the average density is unchanged: at level
+2 the stream goes from 444 one-tick bursts to 147 three-tick bursts, same
+density to four decimals. Under this model `$35` stops being a duty floor
+and becomes a density floor - the control that maps S onto the band that
+does useful work, which is what the factory does with its own scale. Structurally the model is a
 mask on the core's fire state and never a source of one, so emission stays
 exactly where the core commanded it.
 
@@ -1083,8 +1090,35 @@ Open items only. Anything closed is in `CAMPAIGN-LOG.md`.
     under both models - the motion grid is identical and every density FIRE
     tick is one the analog run also fired, so the model only ever masks.
 
-    Owed: one bench drill to choose the base period, which is the parameter
-    the host cannot answer. The factory never emits a pulse shorter than
+    The base period is settled: four bench ladders (F300 at periods 20, 40
+    and 10, then F100 at 20) put the same six rungs on the material every
+    time, and the matched pairs across periods separate the variables - at
+    identical pulse length, halving density killed the mark; at identical
+    density, varying pulse length 3x changed nothing. Feed did not move it
+    either: 10 % at F100 carries 44 % more energy per millimeter than 20 %
+    at F300, which marks, and still left nothing. So the low-end marking
+    limit is average power, not dose per length and not pulse length, and
+    the period can be chosen on other grounds. It stays at 20.
+
+    What the ladders did expose is a floor of our own making, since fixed:
+    at 5 % the model emitted 36 us stubs and the `hv_current` trace shows
+    **no discharge at all** for that rung, while 10 % drew current for its
+    full 15 s and simply marked nothing. The factory never emits below one
+    100 us tick, and reaches low density by skipping windows instead -
+    which is now what `laser_pulse_min_ticks` does.
+
+    Owed: the user-facing scale. The factory maps its whole 1-100 power
+    scale onto density 18.9-79.5 % (fit from the three captures; Full Power
+    is off that line at ~99.7 %), so its "1 %" is the bottom of the band
+    that does useful work rather than 1 % of the physical range. Under the
+    density model `$35` and `$36` are exactly that control - a density
+    floor and ceiling - so the scale is a settings choice, not new code.
+    The floor's value wants one more ladder: the step from 10 % to 20 % is
+    coarse, and the factory's own answer is 18.9 %. Note the captures also
+    run 6.5-18.8 % density on other jobs, so that intercept is a product
+    decision about cutting, not a physical limit - which is why the
+    minimum-pulse fix matters for the raster low end regardless of where
+    the cut scale starts. The factory never emits a pulse shorter than
     100 us; a tick here is 35.5 us, and every pulse restarts the discharge,
     so each carries the strike transient the threshold ladder made visible
     - dose per pulse is therefore probably not proportional to pulse length