ConstA probe moves toward its target and stops on contact, at a point a previewer cannot know. A probe below the origin plane from a known Z at or above it is assumed to trigger at physical Z0 (the material top on the origin plane), which makes the G92 re-zeroes that follow a touch-off converge each cycle exactly like on a real machine, and keeps a repeated probe without a lift sitting on the plane instead of diving through it (see #437). Other probes — including from an un-homed Z, whose real position is unknown — render as a plain travel to the commanded target. The trigger is assumed on the Z axis only; X/Y targets are kept as commanded. A G31 carrying a P word is RepRapFirmware's set-trigger-values form, not a move, and is ignored; one without any axis words (e.g. Marlin's dock-sled G31) is also ignored. The G38 variants only differ in probe direction and error semantics (G38.4/G38.5 probe away from the workpiece; the odd variants tolerate a missed trigger), neither of which a preview can observe, so all of them share this behavior.
Executes a straight probe command (G31, G38.2-G38.5)