What is the ruling on partial anal penetration (such as half or just the tip) without the man reaching what he would reach through vaginal penetration?

The ruling on a man fondling his wife in the anus without penetration. The legal act of intercourse is defined by the entry of the glans or its equivalent. If this occurs, then the forbidden act has taken place. The questioner's phrase "as if half of it or its head enters" is certainly included in the forbidden. One should avoid this area because Satan beautifies the forbidden. And for one who cannot control his sexual urge, approaching this place is forbidden to him, because whatever leads to the forbidden takes its ruling.