fix(ci): skip Rust aggregator when cargo not available in swarm test

The IDF container doesn't have Rust installed. Check for cargo
with shutil.which() before attempting to spawn the aggregator,
falling back to aggregator-less mode (QEMU nodes still boot and
exercise the firmware pipeline).

Co-Authored-By: claude-flow <ruv@ruv.net>

scripts/qemu_swarm.py

@@ -502,10 +502,14 @@ def start_aggregator(
     port: int, n_nodes: int, output_file: Path, log_file: Path
 ) -> Optional[subprocess.Popen]:
     """Start the Rust aggregator binary. Returns Popen or None on failure."""
+    import shutil
     cargo_toml = RUST_DIR / "Cargo.toml"
     if not cargo_toml.exists():
         warn(f"Rust workspace not found at {RUST_DIR}; skipping aggregator.")
         return None
+    if shutil.which("cargo") is None:
+        warn("cargo not found; skipping aggregator (Rust not installed).")
+        return None
 
     args = [
         "cargo", "run",