- Introduce raise_z_above to move Z carefully when the current position
  is potentially unknown, using stallguard
- Use raise_z_above for:
  * filament loading/unloading clearance
  * extruder spacing when preheating (to avoid buildplate marks on PEI)
  * before homing to avoid damaging the build plate and to avoid
    repeated Z moves as well

Since raise_z_above is conditional, it will only raise when needed.

Calling raise_z_above when the extruder position is unknown and already
at maximum travel is safe and will prevent further vertical moves.

In raise_z_above, power Z steppers in normal mode to overcome initial
load. This avoids triggering the endstop right at the start if the
Z axis was started directly in low-power mode.