split the wallet's chain reads from its signing half

OnchainBackend depended on the concrete EsploraChain class, so the only wallet
it could ever have was an Esplora-backed one. The seam is now WalletChainSource,
and it is drawn at the scan rather than at the HTTP client: Esplora is
address-level and has to walk the gap limit, NBXplorer is wallet-level and has
no per-address endpoint at all, so there is nothing to share one level down.

The backend keeps the keys and the money — derivation, snapshot cache, coin
selection, PSBT construction, signing — and owns no HTTP. Which indexer answers
is a constructor argument.

Also adds the NBXplorer implementation of the seam, verified end to end against
the owner's own pruned node, and the first tests over any of this: a stub
Esplora drives a real backend through the gap-limit walk, balance summation,
UTXO mapping, transaction scoring and address issuance. Nothing covered the
scan before it was moved, which is the wrong time to have no tests.

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

src/servers/sidecar/wallet/resolve.ts

@@ -8,6 +8,7 @@ import {
   type WalletSummary,
 } from 'officerdb';
 import { EsploraChain } from './chain';
+import { EsploraChainSource } from './chain-source-esplora';
 import { LndBackend } from './backends/lnd';
 import { ClnRestBackend } from './backends/clnrest';
 import { LndHubBackend } from './backends/lndhub';
@@ -127,7 +128,10 @@ function build(wallet: WalletSummary, config: Record<string, unknown> | null, es
         throw new BackendError('wallet has no account xpubs', 500, 'BAD_CONFIG');
       }
       return new OnchainBackend({
-        chain: new EsploraChain({ baseUrl: esploraUrl, network }),
+        chain: new EsploraChainSource({
+          chain: new EsploraChain({ baseUrl: esploraUrl, network }),
+          label: `esplora(${hostOf(esploraUrl)})`,
+        }),
         network,
         accountXpub,
         // The session is the signer. While locked it holds no key material, so watch-only reads below
@@ -141,3 +145,12 @@ function build(wallet: WalletSummary, config: Record<string, unknown> | null, es
       throw new BackendError(`unknown wallet kind "${wallet.kind}"`, 400, 'BAD_CONFIG');
   }
 }
+
+/** Host only, for a chain-source label. A malformed URL is labelled with itself rather than throwing. */
+function hostOf(url: string): string {
+  try {
+    return new URL(url).host;
+  } catch {
+    return url;
+  }
+}