arc: reth client + pruned/archive profiles; drop snapshot-download init container

- EL client renamed arc -> reth (it IS a reth fork); template arc.yml -> reth.arc.yml
  so find_template picks it as the arc-network reth override. Path is now
  arc/reth/arc-testnet-reth-{pruned,archive}-trace.yml (was arc/arc/arc-testnet-arc-full).
- profiles: full -> pruned[trace] + archive[trace] (there is no 'full' node type).
- removed the {{short_name}}-snapshots init container (arc-snapshots download, ~84 GB) and
  the consensus-init depends_on that pointed at it. Chain data is seeded OUT-OF-BAND, never
  via a compose service. Services: arc-testnet (EL) / arc-testnet-node (malachite CL) +
  consensus-init / sockets-init.
- git rm orphaned arc/arc/arc-testnet-arc-full.yml.

compose_registry.json

@@ -882,7 +882,7 @@
     "compose_file": "arc/arc/arc-testnet-arc-full",
     "features": [],
     "network": "arc",
-    "node": "malachite",
+    "node": null,
     "relay": null,
     "stack": null,
     "type": "full",
@@ -892,6 +892,42 @@
       "arc-testnet-arc-full-sockets"
     ]
   },
+  {
+    "chain": "testnet",
+    "client": "reth",
+    "compose_file": "arc/reth/arc-testnet-reth-archive-trace",
+    "features": [
+      "trace"
+    ],
+    "network": "arc",
+    "node": "malachite",
+    "relay": null,
+    "stack": null,
+    "type": "archive",
+    "volumes": [
+      "arc-testnet-reth-archive-trace-consensus",
+      "arc-testnet-reth-archive-trace-execution",
+      "arc-testnet-reth-archive-trace-sockets"
+    ]
+  },
+  {
+    "chain": "testnet",
+    "client": "reth",
+    "compose_file": "arc/reth/arc-testnet-reth-pruned-trace",
+    "features": [
+      "trace"
+    ],
+    "network": "arc",
+    "node": "malachite",
+    "relay": null,
+    "stack": null,
+    "type": "pruned",
+    "volumes": [
+      "arc-testnet-reth-pruned-trace-consensus",
+      "arc-testnet-reth-pruned-trace-execution",
+      "arc-testnet-reth-pruned-trace-sockets"
+    ]
+  },
   {
     "chain": "fuji",
     "client": "go",