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.

README.md

@@ -128,8 +128,27 @@ All API methods accept a `context.Context` as their first parameter for timeout
 - `Create(ctx, req)` - Create a new transaction
 - `CreateBulk(ctx, req)` - Create multiple transactions
 - `Get(ctx, transactionID)` - Get transaction by ID
+- `GetInterchain(ctx, transactionID, opts...)` - Trace a transaction's interchain anchors
 - `List(ctx)` - List all transactions
 
+#### Interchain trace options
+
+`Transaction.GetInterchain` and `Block.GetInterchain` trace a prime block to the
+public-chain anchors covering it. By default they return the **first anchor per
+chain** (anchor proofs are chained, so the earliest per chain is the meaningful
+one). Tune with functional options:
+
+```go
+// Default: first anchor per chain (ETH, BTC, …)
+trace, _ := client.Block.GetInterchain(ctx, "42")
+
+// Up to 3 anchors per chain
+trace, _ = client.Block.GetInterchain(ctx, "42", sdk.WithPerChain(3))
+
+// All anchors, only the ETH-mainnet chain ("1"); "0" = BTC
+trace, _ = client.Block.GetInterchain(ctx, "42", sdk.WithPerChain(0), sdk.WithChains("1"))
+```
+
 ### Transaction Type
 - `Create(ctx, req)` - Create a new transaction type
 - `Get(ctx, txnType)` - Get transaction type by name