Currently `template_structure.xml` defines a <Vision> of 40. However, most structures define different values. Furthermore, many of the structures in the `other/` folder, which inherit directly from `template_structure.xml`, have a <Vision> of 4 (perhaps because one tile is 4×4?), which is roughly the attack range of a melee unit.
* changes `template_structure.xml` vision to 1.
* inserts a vision of 40 in those files that used to inherit it.
* purges the vision of those other/ structures (this does not alter gameplay balance)
* removes 1 vision lines that are now unnecessary in other structures (outpost, palisades, walls, monument, pillar, arch)
A vision of 1 means the structure can't see its surroundings, but can be selected (unlike fields, which have 0 vision); this seems more generic than the arbitrary value of 40.
* [EDIT] Changed 1 to 2, because players are unable to see capture points or set rally points with a vision of 1 (spotted by @Freagarach).
See also D2505/rP23934 and D3016/rP24104.