server: fix Java explosion knockback

[HashimTheArab]

Summary

• affect entities whose feet coincide with the explosion origin
• calculate explosion impulse from the entity eye position and add it to existing velocity
• safely handle zero-length directions for non-eyed entities
• apply Java-style Blast Protection resistance and preserve consecutive same-tick impulses

Verification

• go test ./...
• make lint
• codex review --uncommitted