When exiting between floors, exit location should be unique per-stairwell

[aebrer]

Currently:

• Player goes from lvl 0 -> lvl 1
• Player appears in a fixed position
• staircase is gone
• find a new staircase back to lvl 0
• appear in the same initial starting position as when moving from lvl -1 -> lvl 0

Intended:

• Player goes from lvl 0 -> lvl 1
• stairs remain behind player
• player can go up the stairs again and return to the same position in lvl 0
• player finds new stairs up in lvl 1
• goes up
• player is in a new location in lvl 0
• corruption is tracked PER LEVEL, and only sums for final score calculation
• corruption persists, when coming back it will still be what it was when the player left
• this design is more interesting and makes multi-level exploration much more meaningful and strategic

[aebrer]

exception:

the stairs up may not be present for special cases: eg. the hole from lvl -1 to lvl 0 (can't return to lvl -1), or future "hole" type exits that won't allow easy climbing back up