Fixes#22
Extracts shared grounded collision/friction and airborne movement/landing policies while preserving state-specific behavior. Running fighters now pass `can_walk_off = true` so they can run off platform edges. Includes grounded and airborne hitstun call sites.
Reviewed-on: #36
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>
EIREXE