arc: Malachite CL is now a proper node component (arc-testnet-node)

Consensus service renamed arc-testnet-consensus -> arc-testnet-node: it is now
generated by templates/nodes/malachite.yml (node: malachite) instead of being
hacked into the arc client template's indexer block. Container body unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

compose_registry.json

@@ -797,7 +797,7 @@
     "compose_file": "arc/arc/arc-testnet-arc-full",
     "features": [],
     "network": "arc",
-    "node": null,
+    "node": "malachite",
     "relay": null,
     "stack": null,
     "type": "full",