Shrinking the roster / deselecting a class leaves stale membership that FillRound still seats

[ryan-johnson2]

From the 2026-07-03 release review (confirmed, serious — validation dimension). events.rs set_event_roster/set_event_classes do not prune classes_membership, so a pilot removed from the roster (or a whole deselected class) can still be seated by FillRound — bypassing the #330 membership guard which only checks at membership PUT time. Prune membership on roster/class shrink (or re-validate at fill).

[ryan-johnson2]

Fixed in #346: roster PUT, per-pilot DELETE, and classes PUT now prune stale membership (shared prune_membership_to_roster); survivors keep channels. Tests: roster_shrink_prunes_the_departed_pilots_membership, removing_a_roster_pilot_prunes_their_membership, class_deselect_prunes_its_membership.