Hear me out on this one.
The "active" flag is intended as a temporary workaround to disable an obstruction. It's currently used by turrets to de-activate the turreted obstructions.
There is another mechanism that does something somewhat similar, but can only disable "block movement" and "block pathfinding". That's used for gates.
It seems like these would probably be somewhat redundant. They're not.
The 'trick' is that gates (resp. foundations) get a list of 'colliding entities' to know if they should open/close (resp. be built). These entities are basically intended to be moving units. For gates, we want to consider stuff that moves/blocks movement (TBH those two concepts aren't really the same, but close enough right now). For foundations, it's kind of arbitrary, but there's basically three kinds of entities: those that can move, those that are deleted on construction (and don't block movement), and those that block foundations in the first place.
This is all a little weird, and has led to bugs before. Namely, D1445 / rP21702 made structures not block construction, only placing foundations, to work around an issue with walls on skirmish maps. I had a similar issue with skirmish-map-walls in rP23710.
The fact of the moment is that we don't really need this "block construction" flag. It's arbitrary. It's also 1-1 with "block movement" for units. So foundations can just check for units that block movements, and gates too.
This also fixes a bug with gates (unsure when it was introduced TBH, there's been changes there), where units that ungarrison from gates no longer properly open gates.
With that, we can also remove the "active" bit.
NB: the reason this diff exists it that' I'm trying something to clean up D14 down the line.