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.

client/interchain.go

@@ -0,0 +1,57 @@
+package client
+
+import (
+	"net/url"
+	"strconv"
+	"strings"
+)
+
+// InterchainOption configures an interchain-trace request (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.
+type InterchainOption func(*interchainConfig)
+
+type interchainConfig struct {
+	perChain *int
+	chains   []string
+}
+
+// WithPerChain caps how many interchain anchors are returned per public chain,
+// earliest-first: 1 is the first anchor per chain (the service default), 0
+// returns all anchors for each chain.
+func WithPerChain(n int) InterchainOption {
+	return func(c *interchainConfig) { c.perChain = &n }
+}
+
+// WithChains restricts the trace to these interchain chain ids ("1" = ETH
+// mainnet, "0" = BTC; testnet ids differ).
+func WithChains(chains ...string) InterchainOption {
+	return func(c *interchainConfig) { c.chains = chains }
+}
+
+// InterchainQuery 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).
+func InterchainQuery(opts ...InterchainOption) string {
+	cfg := &interchainConfig{}
+	for _, o := range opts {
+		o(cfg)
+	}
+
+	var parts []string
+	if cfg.perChain != nil {
+		parts = append(parts, "perChain="+strconv.Itoa(*cfg.perChain))
+	}
+	if len(cfg.chains) > 0 {
+		escaped := make([]string, len(cfg.chains))
+		for i, c := range cfg.chains {
+			escaped[i] = url.QueryEscape(c)
+		}
+		parts = append(parts, "chains="+strings.Join(escaped, ","))
+	}
+	if len(parts) == 0 {
+		return ""
+	}
+	return "?" + strings.Join(parts, "&")
+}