Player can move up and down stairs while grappled

[Ishindri]

From the forum: you can still move up and down stairs while grabbed by zombies.

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[tvm1]

This is more fundamental. Player (or npc) should go through the whole character::move_effects() when calling game::vertical_move().

There are more occasions where player (or npc) can vertically move when they shouldn't (being downed, webbed, being trapped or crushed, etc.).