[Jakfut]

Summary

Fixes several vanilla mechanics that still assumed the world ends at Y0 and Y255

• Beacons: pyramid layers below Y 0 are counted
• Vines: grow past Y 255 and below Y 1
• Nether portal frames: a frame whose bottom is below Y 0 is recognised
• /spawnpoint and /setworldspawn: accept an explicit Y anywhere in the world's height range
• /spreadplayers: where the target's terrain is loaded, it finds the surface at any height and rejects lava,
  fire and blocked spots, with a bounded search from the column's top through loaded cubes only. Where the column,
  its surface or the space above it is not loaded, the vanilla search runs as before:
  it still lands on unexplored terrain like now (Y 2-257), and there it does not reject lava or fire, as now.
• Bed packet: the bed position's Y is sent as a full int instead of a byte
• Pistons (vanilla ones): can push and pull across Y 0 and up to the world's top
• Respawn on the client: when the spawn position is inside blocks below Y 0, the client moves the player up out
  of them like vanilla does above Y 0.

Checklist

• I have tested this PR in DevEnv
• I have tested this PR in Fullpack
• This PR is in compliance with the GTNH AI Policy
• This PR requires another PR in order to merge