io_uring: export SEND_ZC exposure counters so the race tier and the bench A/B can prove the branch ran

[FumingPower3925]

Both celeris#585 (SEND_ZC fabric A/B) and celeris#587 (race tier on the ZC send-state) are blocked on the same missing witness: nothing in the engine says whether a SEND_ZC was submitted, whether its NOTIF arrived, or whether the inline-egress guard blocked on zcNotifPending. Without it a clean result cannot be told apart from 'the branch never ran'.

Add, under -tags=validation (the validation counter package, next to IouringSQECorruptions): IouringSendZCSubmits, IouringSendZCSubmitsDetached (keyed on cs.h1State.Detached), IouringSendZCNotifs, IouringInlineGuardBlockedZC, IouringZCCompletionWithPendingWrite; and in EngineMetrics (always on, one atomic add per event, never per request): ZCSendsSubmitted, ZCNotifs, InlineBytes, RingBytes. Publish them through the refapp's /debug/vars (probatorium debugvars) so the property loop can declare an I-ENG-IOURING-ZC predicate only when the branch was exercised. Also add -tags=validation to probatorium's raceCompileGoBinary so the race tier's refapps carry them. Extend send_zc_gate_test.go with the counter.