moving piston block entities are missing progressO read, causing desyncs and dupes

[Uhuli]

Fixes #9220

Since the class net.minecraft.world.level.block.piston.PistonMovingBlockEntity is responsible for saving the moving blocks on a flying machine, I took a look there. As far as I could test and reproduce, the root cause is that saveAdditional only persists progressO (previous-tick progress) but not progress (current-tick progress):

output.putFloat("progress", this.progressO);

On load, both fields are set to the same value:

this.progress = input.getFloatOr("progress", 0.0F);
this.progressO = this.progress;

This means up to half a tick of movement is lost every time a chunk unloads and reloads mid-extension. For flying machines crossing chunk boundaries, this causes the desync.

If you instead save just this.progress in the existing progress field, flying machines always strictly cut off at the chunk border:

Proposed fix

Save both values independently:

saveAdditional:

output.putFloat("progress", this.progressO);
output.putFloat("progressCurrent", this.progress);

loadAdditional:

this.progressO = input.getFloatOr("progress", 0.0F);
this.progress = input.getFloatOr("progressCurrent", this.progressO);

This preserves backward compatibility - old saves without progressCurrent fall back to progressO. With this change, the full piston state is restored correctly and the flying machine resumes from the exact position it was at when the chunk unloaded.