fix(pay): wait for note maturity on the real wallet, and stop re-shielding

Two defects the first live mainnet run exposed:

1. Only MockWallet implemented the maturity hook, so against a real wallet
   the flow fired the payment one block after the deposit. Pool notes need
   about ten blocks, so that payment could never land. WalletApiAdapter now
   records the block its shield settled in and polls the chain until the
   notes are spendable, reporting blocks remaining as it waits.

2. shieldedBalance answered "0" when the wallet declined to report, so a
   payer with funds already in the pool shielded a second time: a wasted
   pool fee and an extra public leg. It now answers null for UNKNOWN, and
   the flow pays first in that case, shielding only if the wallet actually
   reports insufficient funds.

30 tests green, including regressions for both; full flow re-verified in
the browser end to end.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>