Currently HTTP 402 payment conditions use a single default balance (satoshi on testnet4). This works well for development and testing — testnet coins have no monetary value, making it safe to experiment with payment flows.

To support a broader range of use cases, the balance check could be extended to match the currency/chain specified in the ACL condition:

"acl:condition": {
  "@type": "PaymentCondition",
  "amount": "10",
  "currency": "tbtc4"
}

The webledger already supports per-currency balances via getBalance(ledger, agent, currency). The upgrade path is:

• ACL condition specifies currency/chain
• Server matches against the correct balance
• Different chains tracked independently
• Server's --pay-mempool-url determines which chain is verified on deposit

This keeps the current single-currency flow as the default, while allowing opt-in to multi-currency for advanced deployments.

Builds on #239 and #241.