Add unspent output lookup

[aeitwoen]

Balance and transaction history cannot identify which Bitcoin output is still spendable. explorers_balance only returns the aggregate, while explorers_tx_history emits normalized transaction summaries without vout; proving that an address still controls a1916e7e...:1 required a separate raw call to /api/address/:address/utxo. That gap matters for custody checks because a positive balance does not name the spendable set.

Add a UTXO capability and explorers_utxos operation for UTXO providers, starting with mempool-compatible APIs. Each row should keep txid, vout, value in the smallest unit, and confirmation metadata. Providers without an unspent-output endpoint should advertise the capability as false rather than infer it from truncated history.

[meridianmindx]

Solution for Issue #33: Add UTXO capability and unspent output lookup

Implemented on branch feat/unspent-output-lookup (commit f4ecc2b).

Implementation Details

1. Types & Interfaces (src/core/types.ts):
   • Added Utxo and UtxoStatus domain types with txid, vout, value (raw smallest unit), valueFormatted (satToCoin), and block confirmation metadata.
   • Added utxos: boolean capability flag to ProviderCapabilities.
   • Updated ExplorerProvider interface with optional getUtxos(address, chain?, options?) method signature.
2. Provider Support (src/providers/mempool.ts):
   • Implemented getUtxos(address, chain, options) querying /api/address/:address/utxo with safe path validation and proper query parameter forwarding (unconfirmed).
   • Declared utxos: true on Mempool provider capabilities; declared utxos: false explicitly across all other 10 providers.
3. CLI, MCP, Pi & OMP Surface:
   • Added explorers utxos <address> command in src/commands/utxos.ts and registered in src/cli.ts & src/cli-args.ts.
   • Added explorers_utxos MCP tool registration in src/mcp.ts.
   • Added explorers_utxos tool definitions in both Pi (packages/pi/extensions/explorers.ts) and OMP (packages/omp/extensions/explorers.ts) extensions.
4. Testing:
   • Added unit test suite in test/unit/mempool.test.ts verifying path formation, normalization of UTXO rows, satoshi-to-BTC decimal conversion, and unconfirmed status flags.

Branch: https://github.com/meridianmindx/explorers/tree/feat/unspent-output-lookup
Commit: meridianmindx@f4ecc2b

Base EVM Payout Address: 0xcFDa9f32d292661740a6d0B4c00867E34c05c56D