qa: poll USDC balance for finalization in multicast settlement test

[packethog]

Summary of Changes

• Poll the USDC balance with require.Eventually in validate_balance_after_pay and validate_balance_after_withdraw instead of a single read, with a 30s timeout and 5s interval.
• Fixes a race between FeedSeatPay/FeedSeatWithdraw (which return after tx confirmation) and GetUSDCBalance at CommitmentFinalized — mainnet finalization lags confirmation by ~13s, so a one-shot finalized read can still see the pre-debit balance.
• PR qa: use CommitmentFinalized for USDC balance check #3542 (commitment bump alone) was insufficient: the failure reproduced immediately at run 24534512863, with before/after balance identical because the finalized-slot view predated the pay tx.
• 5s poll interval chosen to avoid RPC rate limiting.

Testing Verification

• go build -tags=qa ./e2e/... and go vet -tags=qa ./e2e/... pass.
• Run failure signature reproduced: balanceBeforePay=1008431000, balanceAfterPay=1008431000, debit=0, polled one ~115ms after FeedSeatPay returned. With polling, the assertion has up to 30s for the finalized slot to advance past the pay tx.

[karl-dz]

We should investigate this closer once we get QA working