docs: extreme-detail reference for SA core structure

Disciplines, academies/contracts, programs, groups (enroll vs force-enroll,
the three disagreeing pause implementations, medical grace-period sweep),
coaches, facilities/units, pricing resolution algorithm, attendance,
player cards, and both SA reports. Documents the coach-cost fix's real
impact (9 of 25 active coaches previously cost 0 in the finance report)
and flags the card-renewal duration bug as a deliberate fix, not a
quick patch, since it touches both fee calculation and payment payload.