Add s7 demo subcommand: live server exposing real host metrics

A runnable, demoable S7 PLC backed by your own machine. CPU %, memory %,
disk/network throughput, temperature and fan RPM write into well-known
DB1 offsets every couple of seconds; a writable DB2 block lets clients
(e.g. the ha-s7 Home Assistant integration) flip a lamp, set a
brightness, push a text message, and the demo prints each write with a
timestamp. Optional rich live display shows current sensor values and
scrolling write history in a full-screen dashboard.

Runs as `s7 demo --port 10102` after `pip install "python-snap7[cli,demo]"`.
psutil lives behind a new `demo` extra so the core install stays thin.

DB layout is documented in the module docstring (and mirrored in
constants) so users can copy-paste PLC4X tag addresses straight into a
Home Assistant config.

Platform notes: CPU temperature and fan speed report 0 where psutil has
no sensor backend (macOS, most containers) — intentional, not a bug.

Tests exercise the collector, sensor encoder, threshold BOOL derivation,
and control-write diffing without spinning up the server; end-to-end
server coverage already lives in test_s7_unified and would duplicate
work here.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: gijzelaerr

pyproject.toml

@@ -35,6 +35,7 @@ Documentation = "https://python-snap7.readthedocs.io/en/latest/"
 test = ["pytest", "pytest-asyncio", "pytest-cov", "pytest-html", "hypothesis", "mypy", "types-setuptools", "ruff", "tox", "tox-uv", "types-click", "uv"]
 s7commplus = ["cryptography"]
 cli = ["rich", "click" ]
+demo = ["psutil"]
 doc = ["sphinx", "sphinx_rtd_theme"]
 discovery = ["pnio-dcp"]
 

snap7/cli.py

@@ -140,6 +140,41 @@ def server(port: int) -> None:
     mainloop(port, init_standard_values=True)
 
 
+@main.command()
+@click.option("-p", "--port", default=10102, help="Port the server will listen on.")
+@click.option(
+    "-r",
+    "--refresh",
+    default=2.0,
+    type=float,
+    help="Seconds between metric samples.",
+)
+@click.option(
+    "--plain",
+    is_flag=True,
+    help="Use plain log output instead of the rich live display.",
+)
+def demo(port: int, refresh: float, plain: bool) -> None:
+    """Start a live S7 server that exposes real host metrics on DB1.
+
+    Requires the ``demo`` extras (psutil). For the live display, also
+    install the ``cli`` extras (rich):
+
+        pip install "python-snap7[cli,demo]"
+    """
+    try:
+        from snap7.demo import run_demo
+    except ImportError as err:
+        click.echo(f"Failed to load demo module: {err}", err=True)
+        sys.exit(1)
+
+    try:
+        run_demo(port=port, refresh_seconds=refresh, live=not plain)
+    except RuntimeError as err:
+        click.echo(str(err), err=True)
+        sys.exit(1)
+
+
 @main.command()
 @click.argument("host")
 @click.option("--db", required=True, type=int, help="DB number to read from.")