Capture pre-skill-apply XP and perk snapshots; on TryRecord loss reverse
each applied skill row via negative delta and restore the perk snapshot so
level-up unlocks from the loser call are reverted without clobbering a
concurrent winner's XP.
Replace fixture player ids with POST …/__dev/mastery-fixture (resetPerkState,
absolute skillXp). Bruno deny tests set state in pre-request on dev-local-1.
Gated to Development/Testing or Game:EnableMasteryFixtureApi.