Assorted ZAS fixes

[NataKilar]

Description of changes

An assortment of ZAS fixes, mostly related to the creation of unsimulated edges between zones and space.

• Creating a hole in the floor into space will now create an unsimulated edge as expected. This is done by checking if the zone_membership_candidate variable has changed to FALSE during a turf change. Every other case should be covered by the typical checks for exterior atmosphere participation.
• Changed the default open_turf for flooring and natural flooring to airless variants of open space. This fixes a bug where space tiles weren't being removed from Z-stacks when the bottom most space tile was replaced by plating. I haven't tested how this affects issue Multiz shuttles replace open turfs with space. #3999 yet.
• Doors and other atoms that need to update ZAS connections will now update their loc's neighbors if the atom is located on top of an exterior turf. This fixes a bug where unsimulated edges weren't being updated or created as expected Supplanted by new update code.
• Fixes a longstanding issue where connections for ZAS edges in the up and down direction weren't being added to turfs' connection managers

New Changes 06/08/2024

• You can now safely call SSair.mark_for_update on non-ZAS participating turfs to create the appropriate unsimulated edges
• Improved consistency of updating exterior air participation. Newly ZAS participating turfs will now only assume exterior air if they were previously outside
• Adds keep_air = TRUE to a variety of ChangeTurf calls where appropriate
• Elevators no longer move air by default.
• Shuttles now create roofs at their destination before moving, and only remove the roof after they have moved.

As a consequence of these changes, keep_air_below has been cut entirely.

Why and what will this PR improve

Fixes a variety of ZAS issues stemming from the simulated/unsimulated turf merger. There is still an issue with gas conservation when merging two zones between z-levels, which I still need to resolve.

Authorship

Myself. Thanks to Penny for the multi-Z get_dir code.

Changelog

🆑
bugfix: Fixed space tiles not creating vacuums in certain conditions
/:cl:

[MistakeNot4892]

Fixes all look good to me and you justified/explained the ones I was confused about on Discord, so looks fine to me.

Bug with elevators

[MistakeNot4892]

Per Discord, issues with elevators need to be sorted out.