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).

gateway/internal/transcode/transcode.go

@@ -382,11 +382,11 @@ func walletOrderHandler(backend *backendclient.Client, minter InvoiceMinter) Han
 }
 
 // walletRewardHandler credits a watched rewarded video and returns the updated wallet. The nonce is
-// the client's per-view idempotency key; diag carries the raw VK ad result for the contour diagnostic.
+// the client's per-view idempotency key.
 func walletRewardHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsWalletRewardRequest(req.Payload, 0)
-		w, err := backend.WalletReward(ctx, req.UserID, string(in.Nonce()), string(in.Diag()))
+		w, err := backend.WalletReward(ctx, req.UserID, string(in.Nonce()))
 		if err != nil {
 			return nil, err
 		}