Add Arc testnet node (Circle L1): arc-execution EL + arc-consensus CL

Generated compose for the new `arc` client. Single self-contained stack:
3 init containers (snapshot download ~84GB, CL key init, socket prep) +
EL (reth-fork, http RPC behind traefik /arc-testnet, dshackle upstream
chain: arc-testnet) + CL (follows relay endpoints, drives EL over shared
IPC sockets). Images pulled anonymously from Circle's public Cloudsmith
registry at 0.7.2. Positional subcommands pinned in entrypoint so
process_yaml's command-sorting can't reorder them.

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

compose_registry.json

@@ -791,6 +791,22 @@
       "arbitrum-sepolia-nitro-erigon-pruned-trace"
     ]
   },
+  {
+    "chain": "testnet",
+    "client": "arc",
+    "compose_file": "arc/arc/arc-testnet-arc-full",
+    "features": [],
+    "network": "arc",
+    "node": null,
+    "relay": null,
+    "stack": null,
+    "type": "full",
+    "volumes": [
+      "arc-testnet-arc-full-consensus",
+      "arc-testnet-arc-full-execution",
+      "arc-testnet-arc-full-sockets"
+    ]
+  },
   {
     "chain": "fuji",
     "client": "go",