GetInterchain: perChain + chains options (default first anchor per chain)

Transaction.GetInterchain and Block.GetInterchain take variadic options
(client.WithPerChain / client.WithChains, re-exported as sdk.WithPerChain /
sdk.WithChains) that map to the new prime-node ?perChain=&chains= query params.
Backward compatible: existing no-option calls get the default (one anchor per
chain). client.InterchainQuery builds the suffix; unit-tested.

dragonchain.go

@@ -113,3 +113,15 @@ func NewDragonchainSDKWithHTTPClient(publicID, authKeyID, authKey, baseURL strin
 func (sdk *DragonchainSDK) GetClient() *client.Client {
 	return sdk.client
 }
+
+// InterchainOption configures Transaction.GetInterchain / Block.GetInterchain.
+// Re-exported from the client package for convenience (e.g. sdk.WithPerChain).
+type InterchainOption = client.InterchainOption
+
+// WithPerChain caps interchain anchors returned per public chain (1 = first per
+// chain, the default; 0 = all). See client.WithPerChain.
+var WithPerChain = client.WithPerChain
+
+// WithChains restricts an interchain trace to specific chain ids ("1" = ETH
+// mainnet, "0" = BTC). See client.WithChains.
+var WithChains = client.WithChains