containers: Docker (and best-effort Podman) plugin

- minimal Docker Engine API client over Unix/TCP sockets with version
  negotiation and chunked response decoding
- drop-in stats schema: active/stopped containers, CPU/memory/IO/network
  rates, ports, uptime (pretty_date), image, engine field
- Docker + Podman engines, config (all, podman_sock, max_name_size,
  disable_stats, show/hide, per-container thresholds)
- export strips the computation sub-dicts (--stdout/--stdout-csv)
- TUI block above the process count
- CI-safe fake Docker socket test + live differential parity

tools/parity.py

@@ -159,12 +159,15 @@ def main():
     plugins = args.plugins or DEFAULT_PLUGINS
     errors = []
     for plugin in plugins:
+        # The containers plugin needs a couple of refreshes for its rates to
+        # become available (both implementations warm up their second sample).
+        iterations = max(args.iterations, 6) if plugin == "containers" else args.iterations
         extra = ["-C", args.config] if args.config else []
-        rust, rust_error = run(args.rust, [plugin], args.iterations, extra)
+        rust, rust_error = run(args.rust, [plugin], iterations, extra)
         if rust_error is not None:
             errors.append(f"{plugin}: Rust run failed: {rust_error}")
             continue
-        python, python_error = run_python(args.python, args.config, [plugin], args.iterations, [])
+        python, python_error = run_python(args.python, args.config, [plugin], iterations, [])
         if python_error is not None:
             errors.append(f"{plugin}: Python run failed: {python_error}")
             continue