Add Liquid address asset balance helper

[Copilot]

Viewing Liquid assets by address required stitching together UTXOs and asset metadata manually, and the current address typings did not expose an address-level balance helper. This change adds a first-class API for deriving per-asset balances directly from a Liquid address.

• API

  • Adds liquid.addresses.getAddressAssetBalances({ address })
  • Returns grouped balances per asset with:
    • asset_id
    • value
    • utxo_count

• Liquid typing updates

  • Introduces Liquid-specific address interfaces instead of reusing the Bitcoin address types
  • Extends Liquid transaction outputs with asset so asset ids can be resolved from UTXOs

• Balance aggregation

  • Aggregates unspent outputs by asset id for a given address
  • Reuses /address/{address}/utxo and resolves output asset ids from the referenced transactions
  • Deduplicates transaction fetches before grouping balances

• Docs and examples

  • Documents the new helper in README-liquid.md
  • Updates Liquid Node.js and HTML address examples to show address-level asset balance retrieval

const {
  liquid: { addresses },
} = mempoolJS({ hostname: 'liquid.network' });

const balances = await addresses.getAddressAssetBalances({
  address: 'YOUR_ADDRESS',
});

console.log(balances);