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/scrabble.fbs

@@ -877,10 +877,9 @@ table WalletBuyRequest {
 }
 
 // WalletRewardRequest credits a watched rewarded video: nonce is the per-view idempotency key (a
-// retry credits once); diag carries the raw VK ad result for a temporary contour diagnostic log.
+// retry credits once).
 table WalletRewardRequest {
   nonce:string;
-  diag:string;
 }
 
 // WalletOrderRequest opens a money order to fund a chip pack: the pack to buy.