Switch esplora/electrum defaults

[Kukks]

arkade-os/ts-sdk#461

[arkanaai]

Context

Mirrors the drive-by fix in ts-sdk#461. Two related problems in the go-sdk:

1. verbose=true breaks electrs compatibility

File: explorer/electrum/explorer.go:708

func (e *explorerSvc) getVerboseTx(txid string) (*electrumVerboseTx, error) {
    result, err := e.client.request("blockchain.transaction.get", []any{txid, true}) // ← electrs rejects this

getVerboseTx is called by:

• GetTxs (line 288) — relies on verbose fields (Vin[].Prevout, Vout[].Value, Blocktime)
• GetTxOutspends (line 310) — uses Vout from verbose response

electrs returns "verbose transactions are currently unsupported". Only Fulcrum supports it. This means the electrum explorer silently fails against electrs-based public nodes (Blockstream, mempool.space Electrum, self-hosted electrs).

2. No default electrum endpoints

File: init.go:19-26

var defaultExplorerUrl = map[string]string{
    arklib.Bitcoin.Name:        "https://mempool.space/api",       // esplora only
    arklib.BitcoinTestNet.Name: "https://mempool.space/testnet/api",
    arklib.BitcoinSigNet.Name:  "https://mempool.space/signet/api",
    // ...
}

No electrum defaults exist. WithElectrumExplorer always requires the caller to supply a URL; there's no fallback.

---

Acceptance Criteria

• GetTxs and GetTxOutspends work against electrs (no verbose=true dependency)
  • Parse raw hex via blockchain.transaction.get with false, decode with btcd/wire
  • Derive input amounts from prior tx raw hex (fetch prevout txs), or omit if not needed by callers
  • Block timestamps via blockchain.block.header (already implemented at line 718)
• Add default electrum endpoints to defaultExplorerUrl or a parallel defaultElectrumUrl map
  • Mainnet: ssl://electrum.blockstream.info:50002 or ssl://mempool.space:50002
  • Testnet/signet: equivalent public electrum nodes
  • Regtest: tcp://127.0.0.1:50001 (nigiri default)
• verboseTxToExplorerTx and electrumVerboseTx can be removed or kept for Fulcrum-only opt-in
• Existing electrum e2e tests pass against nigiri (electrs-backed)

Approach

1. Fix verbose dependency — replace getVerboseTx in GetTxs/GetTxOutspends with raw tx parsing. The blockTimestamp helper (line 718) is already correct and reusable.
2. Add electrum defaults — either add a defaultElectrumUrl map and a WithDefaultElectrumExplorer() option, or switch defaultExplorerUrl entries to electrum URLs and adjust newExplorer routing logic accordingly.
3. Add/extend e2e tests — verify the electrum explorer against the nigiri stack (electrs).