Assert zero z clearance for homing again

[DerAndere1]

Description

Re-introduce this sanitycheck which was deleted in 7aaba66

Requirements

#define Z_HOME_DIR -1
#define HOME_Z_FIRST
#define QUICK_HOME

Benefits

Prevent crash into Z max with above config

Configurations

None

Related Issues

#27370

[thisiskeithb]

Pasting my comment here since you may have missed it:

@DerAndere1: If you have a fixed gantry printer with a bed that moves vertically in the Z direction and homes to Z max (bottom of the printer), you'd still want to allow a Z "raise" if homing X or Y to not damage the nozzle or bed.

This would be better suited as a warning (with option to disable) instead of a requirement.

Thinkyhead is reworking Z raises, so we'll see how it all shakes out.

[thinkyhead]

A positive raise is needed to avoid the bed clips or other things when homing X or Y when Z has not yet been homed and (for whatever reason) the nozzle is very close to the bed. This is something we want to permit, so the raise must remain configurable even for the case of a machine that homes first to max Z in response to G28. If the first command issued to the machine is G28 X or G28 Y we want to make sure to provide some clearance value for those edge cases.

Meanwhile, we can work out logic to determine for various kinds of machines when the current_position.z can be trusted to be <= the real position and work off of that information.