fix(payments): VK-iOS freeze is purchase-only; remove the rewarded diagnostic

Testing the rewarded slice surfaced a contradiction: rewarded ads let a VK-iOS
user earn vk chips, but the blanket VK-iOS "spend freeze" then blocked spending
them (the wallet showed "5 (view-only)"). Apple's ToS forbids only BUYING in-app
values on VK-iOS (money -> chips), not spending or earning them — so the freeze is
corrected to purchase-only: vkFrozen() now gates only CreateOrder (the money-in
step), not spendableSources (spending). VK-wallet chips — earned via rewarded ads
or bought on the same account elsewhere (VK Android) — now spend on VK-iOS too.
(Owner's ToS finding.)

Also: the temporary contour diagnostic confirmed VK returns only {result:true} for
a rewarded view (no token/signature) — client-attested is final, no hardening
possible — so the diagnostic (the log and the diag wire field) is removed.

Docs: PAYMENTS(+ru) VK-iOS freeze + D17 amend + PLAN E6. Tests updated (gate /
wallet VK-iOS now spendable).

pkg/fbs/scrabblefb/WalletRewardRequest.go

@@ -49,23 +49,12 @@ func (rcv *WalletRewardRequest) Nonce() []byte {
 	return nil
 }
 
-func (rcv *WalletRewardRequest) Diag() []byte {
-	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
-	if o != 0 {
-		return rcv._tab.ByteVector(o + rcv._tab.Pos)
-	}
-	return nil
-}
-
 func WalletRewardRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(2)
+	builder.StartObject(1)
 }
 func WalletRewardRequestAddNonce(builder *flatbuffers.Builder, nonce flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(nonce), 0)
 }
-func WalletRewardRequestAddDiag(builder *flatbuffers.Builder, diag flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(diag), 0)
-}
 func WalletRewardRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }