getInterchain: perChain + chains options (default first anchor per chain); bump 1.5.0

transaction.getInterchain / block.getInterchain take an optional
{ perChain?, chains? } that maps to prime-node's ?perChain=&chains= params.
Default (no options) returns one anchor per chain. Shared buildInterchainQuery
helper + InterchainOptions type, exported; jest-tested.

src/interchain.ts

@@ -0,0 +1,37 @@
+/**
+ * Options + query builder for the interchain-trace endpoints
+ * (transaction.getInterchain / block.getInterchain).
+ *
+ * Anchor proofs are chained, so by default the trace returns the first anchor
+ * per public chain; these options change how many and which chains are returned.
+ */
+
+export interface InterchainOptions {
+  /**
+   * Max interchain anchors returned per public chain, earliest-first.
+   * 1 = the first anchor per chain (the service default); 0 = all anchors.
+   */
+  perChain?: number;
+  /**
+   * Interchain chain ids to include ("1" = ETH mainnet, "0" = BTC; testnet ids
+   * differ). Omit to include every chain found.
+   */
+  chains?: string[];
+}
+
+/**
+ * Builds the "?perChain=...&chains=..." suffix for the interchain trace
+ * endpoints. Returns "" when no options are set (the server then applies its
+ * defaults: one anchor per chain, all chains).
+ */
+export function buildInterchainQuery(options?: InterchainOptions): string {
+  if (!options) return '';
+  const parts: string[] = [];
+  if (options.perChain !== undefined) {
+    parts.push(`perChain=${encodeURIComponent(String(options.perChain))}`);
+  }
+  if (options.chains && options.chains.length > 0) {
+    parts.push(`chains=${options.chains.map(encodeURIComponent).join(',')}`);
+  }
+  return parts.length ? `?${parts.join('&')}` : '';
+}