Commission the laser duty floor; record how the factory sets power

The pthresh ladder on scrap puts the tube's two thresholds far apart: the
discharge strikes between 2 and 3 percent duty, but nothing lases usefully
below 16 percent (PWMSAR 20), and the rungs between show only a spot at
each line start. $35 ships at 16 (grblhal-glowforge 9466f76, pinned here).

The drill said current lift-off and first mark share a rung; this run
falsifies that, so its docstring and read-the-material text now name both
thresholds and warn that a start-of-line spot is below the threshold, not
at it.

A start-of-line spot is also what a full-power leak at a kernel run start
would look like, so laser_stream_test gains a ladder session (rule 10):
every FIRE tick must ride a commanded duty, and the fire ticks must divide
evenly across rungs. Both hold exactly - six commanded duties, no others,
and 28296 fire ticks on every rung - so the spots are the tube, not the
stream. The harness now derives its expectations from the floor, which
moves the M4 session's S500 plateau from 63 to 73.

laser.power-floor is a new auto acceptance test, the suite's only
non-firing one: a machine must actually carry the commissioned floor,
since stored settings beat freshly baked defaults.

Three cloud cuts of one square at Precision Power 1, 100 and Full Power
show what the analog path is competing with: the power byte is pinned at
127 in all three, dose is FIRE-bit density on a fixed 7-tick period with
the on-count dithered between adjacent integers, and the power setting
never reaches the machine at all. Facts bank and item 17 carry the
numbers; CAMPAIGN-LOG carries both sessions.

docs/BRINGUP.md

@@ -651,6 +651,39 @@ not a release.
 - **Laser PWM**: 39.98 kHz register-verified (divider 13 × 127 counts), scope-
   confirmed at 25.0 µs period across the full duty range, clean at the low end
   (6.4 % measured vs 6.3 % commanded at PWMSAR=8).
+- **Laser duty thresholds** (ladder on scrap at F300, constant power): the
+  tube has two thresholds, far apart. The discharge **strikes between 2 % and
+  3 %** duty — 2 % (PWMSAR 2) draws no measurable `hv_current` and leaves
+  nothing at all, 3 % (PWMSAR 3) draws current — but it does **not lase
+  usefully until 16 %** (PWMSAR 20), the lowest duty leaving a continuous
+  mark. Between them (3–14 %) is a **dead band**: current flows and climbs,
+  and each line shows only a spot at its start (the strike transient) with a
+  dark line after it. So the usable analog range is ~16–100 %, and `$35`
+  (`DEFAULT_SPINDLE_PWM_MIN_VALUE`) ships at **16** to hold every nonzero S
+  above it. Raw `hv_current` counts are a presence/absence witness only: the
+  per-rung means are non-monotonic at the top of the ladder and the signal
+  has no characterized transfer function.
+- **Factory power model** (three cloud cuts of one 1" square, same location,
+  material and speed, only the UI power setting changed; captures in
+  `_RESOURCES/power-settings-20260817/`): the **power byte is pinned at 127**
+  in all three runs — three occurrences each, one as the cut begins and a
+  refresh every ~27 000 ticks (~2.7 s). Analog duty is never a power control.
+  **Dose is FIRE-bit density on a fixed 7-tick period** (700 µs at
+  `STfr` = 10 000, ~1.43 kHz), the on-count dithered between adjacent integers
+  to reach a fractional duty: Precision Power 1 = 1.371 of 7 (density 0.1953,
+  runs of 1 and 2), PP 100 = 5.576 of 7 (0.7952, runs of 5 and 6), Full Power
+  = 7 of 7 (0.9965, continuous). The period was exactly 7 in all 570 measured
+  cycles of both dithered runs, and the mix of adjacent on-counts matches the
+  fractional part exactly (PP 1 wants 1.371; 2-runs are 212 of 571 = 0.371).
+  The three **headers are identical** — the power setting never reaches the
+  machine, so the whole model is service-side. Motion is identical too: 5420
+  steps, 101.62 mm, 10.81 s at 9.44 mm/s. **Density tracks velocity through
+  corners**, by the same relative factor at every power setting (corner/cruise
+  0.38, 0.38, 0.41), but only partly: fire ticks per step rise 3.89 → 7.00 as
+  speed falls 9.44 → 1.22 mm/s, so dose per unit length rises ~1.8× at a
+  corner instead of the ~7.7× it would rise with no compensation. On the UI
+  scale, PP 1→100 is linear in density (~0.006 per unit, intercept ~0.189) and
+  Full Power sits off that line, where PP ~134 would land.
 - **Cooling operating point**: 40 % heater duty, 50 s window, flow-rise
   threshold 14.4 °C, re-checks every 150 s. Below ~40 % duty the stagnant loop
   sheds the heater's output by convection well enough to mimic flow (at 30 %,
@@ -974,33 +1007,43 @@ Open items only. Anything closed is in `CAMPAIGN-LOG.md`.
     modes, 4.1× and 16.4× fewer bytes); frame rate only spaces the stalls out,
     and the existing `FORGECTRL_STREAM_FPS` cap skips demosaic and encode but
     still dequeues every frame. Shares the bench slot with item 8.
-17. **Laser power model and the missing duty floor.** grblHAL maps S onto the
+17. **Laser power model: dose by FIRE-bit density.** grblHAL maps S onto the
     analog PWM duty (`$30`/`$31` → `$35`/`$36`, written raw into PWMSAR against
-    the 127-count period), and ForgeFIRM overrides only `$32`, so a shipped
-    machine has `$35` = 0: duty runs linearly to zero with S and nothing stops
-    it falling below the tube's striking threshold. Under M4 the core scales S
-    by velocity, so every corner, every reversal, and every segment shorter
-    than the accelerate-in-and-out distance (~1.6 mm at 2000 mm/min with the
-    default 700 mm/s²) is commanded below the striking point and does not burn
-    at all.
+    the 127-count period). `$35` now ships at 16, the measured lasing
+    threshold (facts bank), which keeps M4's velocity-scaled power out of the
+    dead band at corners, reversals and segments shorter than the
+    accelerate-in-and-out distance (~1.6 mm at 2000 mm/min with the default
+    700 mm/s²) — where an unfloored duty is commanded below the threshold and
+    does not burn at all.
 
-    The factory does not use duty as a power control. All five firing jobs in
-    the captured pulse files pin the power byte at 127 (one also uses 102) and
-    modulate dose entirely by dithering the FIRE bit at the 10 kHz tick, at
-    6.5–18.8 % density. Two consequences: the captures cannot supply a `$35`
-    default, because nothing in them runs anywhere near the threshold; and the
-    duty → optical-power transfer function of this HV supply is unmeasured,
-    because nothing has ever depended on it.
+    The floor is a patch on a model this tube does not fit. Only 16–100 % of
+    the duty range does anything, so analog control has a ~6:1 span, and the
+    floor buys freedom from dropout by putting its full 16 % into corners
+    where velocity — and dose per unit length — goes the other way. The
+    factory does not use duty as a power control at all: all five firing jobs
+    in the captured pulse files pin the power byte at 127 (one also uses 102)
+    and modulate dose entirely by dithering the FIRE bit at the 10 kHz tick,
+    at 6.5–18.8 % density. The measured dead band is why. Dose set by pulse
+    density cannot fall below the lasing threshold by construction, which is
+    what the per-tick FIRE bit exists for, and it is the only power model this
+    tube and supply are known to work well with. The duty → optical-power
+    transfer function is still unmeasured — nothing has ever depended on it.
 
-    Owed, in order: run `live_fire_drills.py pthresh` on scrap with `$35` = 0
-    to find the striking threshold, set `DEFAULT_SPINDLE_PWM_MIN_VALUE` (a
-    percent) in `grblHAL-glowforge/src/boards/glowforge.h` from it — the
-    marking rung's percent is the value — and record the number here. Then the
-    design question behind it: whether to follow the factory and modulate dose
-    by FIRE-bit density at a fixed high duty rather than by analog duty. That
-    is what the per-tick FIRE bit exists for, it cannot fall below the striking
-    threshold by construction, and it is the only power model this tube and
-    supply are known to work well with.
+    The factory's implementation is now measured rather than inferred (facts
+    bank): power byte pinned at 127, dose set by a fixed 7-tick period
+    (~1.43 kHz) whose on-count is dithered between adjacent integers, and a
+    velocity compensation that is real but partial. Two things follow for the
+    ForgeFIRM implementation. The base period is a free parameter — the
+    factory's 700 µs is 7 ticks at its 10 kHz print rate, and GRBL mode ships
+    the stream at 28 160 Hz, so the same PRF is ~20 ticks; the accumulator, not
+    the period, is what recovers fractional density. And velocity scaling
+    arrives for free: under M4 the core already scales S by velocity, so
+    mapping S onto density inherits compensation that is *more* complete than
+    the factory's, which still lets dose per unit length rise ~1.8× at a
+    corner.
+
+    Owed: the density model itself. `$35` and the analog path stay as the
+    fallback until it lands.
 
     What that model means for image engraving, since it decides the design as
     much as cutting does. LightBurn has two image paths. Its 1-bit modes