get_interchain: per_chain + chains options (default first anchor per chain); bump 0.5.0

transaction.get_interchain / block.get_interchain take per_chain= and chains=
kwargs mapping to prime-node's ?perChain=&chains= params. Default (no kwargs)
returns one anchor per chain. Shared interchain_query() helper, exported; pytest.

README.md

@@ -151,6 +151,24 @@ client = DragonchainSDK(
 - `proof_measure.report(req)` — Per-transaction "securedBy" report over interchain anchors.
 - `proof_measure.health()` — Service liveness.
 
+## Interchain trace
+
+`transaction.get_interchain` / `block.get_interchain` 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):
+
+```python
+# Default: first anchor per chain (ETH, BTC, …)
+trace = client.block.get_interchain("42")
+
+# Up to 3 anchors per chain
+trace = client.block.get_interchain("42", per_chain=3)
+
+# All anchors, only the ETH-mainnet chain ("1"); "0" = BTC
+trace = client.block.get_interchain("42", per_chain=0, chains=["1"])
+```
+
 ## Proof Measure
 
 `proof-measure` is a separate, **public, unauthenticated** Dragonchain service