arc: add snapshot-init wrapper for testnet pruned bootstrap

Shell entrypoint wraps arc-snapshots so compose can optionally override EL/CL
snapshot URLs via ARC_SNAPSHOT_* env vars while defaulting to latest from
snapshots.arc.network on fresh volumes.

arc/scripts/snapshot-init.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+set -eu
+
+CHAIN="${ARC_SNAPSHOT_CHAIN:-arc-testnet}"
+EXEC_PATH="${ARC_SNAPSHOT_EXECUTION_PATH:-/data/execution}"
+CONS_PATH="${ARC_SNAPSHOT_CONSENSUS_PATH:-/data/consensus}"
+
+if [ -n "${ARC_SNAPSHOT_EXECUTION_URL:-}" ] && [ -n "${ARC_SNAPSHOT_CONSENSUS_URL:-}" ]; then
+  exec /usr/local/bin/arc-snapshots download \
+    --chain="${CHAIN}" \
+    --execution-path="${EXEC_PATH}" \
+    --consensus-path="${CONS_PATH}" \
+    --execution-url="${ARC_SNAPSHOT_EXECUTION_URL}" \
+    --consensus-url="${ARC_SNAPSHOT_CONSENSUS_URL}"
+elif [ -n "${ARC_SNAPSHOT_EXECUTION_URL:-}" ] || [ -n "${ARC_SNAPSHOT_CONSENSUS_URL:-}" ]; then
+  echo "Both ARC_SNAPSHOT_EXECUTION_URL and ARC_SNAPSHOT_CONSENSUS_URL must be set together" >&2
+  exit 1
+else
+  exec /usr/local/bin/arc-snapshots download \
+    --chain="${CHAIN}" \
+    --execution-path="${EXEC_PATH}" \
+    --consensus-path="${CONS_PATH}"
+fi