`plotinfo.site_id` is -1 in a reclaimed fortress - can we safely mitigate?

[ab9rf]

should we have a tool that forcibly updates plotinfo.site_id when it's found to be -1 in fort mode (as this violates what we believe to be an invariant) as a mitigation of a DF bug - at least until Bay12 fixes it

Originally posted by @ab9rf in #5923 (review)

[Alistair-Afton]

Implemented in #5948: quickfix.repair_site_id() runs from onMapLoad.default.init and restores site_id from plotinfo.main.fortress_site when DF leaves it at -1. It only writes when the invariant is violated and prints a note when it repairs something.