Prove the corner rolloff; document the one-press recorder

The stream harness gains rule 20: under M4 with the curve in force, the
accelerate-in head of a line renders less density at the default gamma
of 2 than at gamma 1 (0.317 against 0.537 on the host grid) while the
cruise middle renders identically - the exponent shapes only the
velocity-scaled rolloff, never the programmed level. The panel-serves
covers widen to the whole recorder (src/curverec.*). BRINGUP describes
the rolloff and the one-press recorder; the MOTION table gains
laser_corner_gamma; the panel help explains the knob.

docs/MOTION.md

@@ -428,6 +428,7 @@ only be changed while the machine is idle.
 | `laser_disarm_s` | 60 | Spindle-off grace before the armed window closes. |
 | `laser_floor_density` | 10 | The S-range floor, percent of full: the lowest pulse density that still marks. Loaded into `$35` at every spindle precompute; `$35` is derived, never typed. |
 | `laser_dose_curve` | (bench default) | The measured dose curve as density:light percent pairs; S commands a light fraction and the driver maps it onto the density that delivers it. `off` = identity; a bad value falls back to the default. The panel's recorder measures and applies a machine's own. |
+| `laser_corner_gamma` | 2 | The corner rolloff under M4: delivered light follows (v/v_programmed)^gamma, so 1 is plain proportionality and higher values starve the slow spots where heat accumulates. Rides the curve. |
 | `laser_pulse_ticks` | 20 | Density base period in machine ticks (35.5 us each). |
 | `laser_pulse_min_ticks` | 3 | Shortest density pulse in ticks; below it a period is skipped and its debt carried. |
 | `rail_settle_s` | 2.5 | Motor-rail off period when a controller takes the device standalone. |