fix(marmot-interop): guard empty-array expansion in snapshot_invites

bash 3.2 (stock on macOS) raises "unbound variable" on "${gids[*]}"
when the array is empty under `set -u`. The happy-path sanity check
still worked (the array was populated), but the shell error aborted
execution before the actual test cases ran on a fresh state/ directory.

Only expand when the array has at least one entry.

tools/marmot-interop/lib.sh

@@ -198,7 +198,12 @@ snapshot_invites() {
         g=$(printf '%s' "$one" | jq_group_id)
         [[ -n "$g" ]] && gids+=("$g")
     done < <(printf '%s' "$raw" | jq -c '(.result // .) | .[]?' 2>/dev/null)
-    local IFS=','; printf '%s' "${gids[*]}"
+    # bash 3.2 (stock macOS) treats "${gids[*]}" on an empty array as an
+    # unbound reference under `set -u`, so guard the expansion.
+    if (( ${#gids[@]} > 0 )); then
+        local IFS=','
+        printf '%s' "${gids[*]}"
+    fi
 }
 
 # wait_for_invite <B|C> <timeout-seconds> [ignore-csv]