Add s7 demo subcommand: live server exposing real host metrics

[gijzelaerr]

Summary

A runnable, demoable S7 PLC backed by your own machine. Real 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 flip a lamp, set a brightness, push a text message, and the demo prints each write with a timestamp.

pip install "python-snap7[cli,demo]"
s7 demo --port 10102

Optional rich live display shows a full-screen dashboard of current sensor values plus a scrolling write history. Falls back to plain periodic logging when rich isn't installed.

Why

Up to now there was no way to see python-snap7 do anything without wiring up a real PLC or writing test-harness code. This gives:

• A README GIF path — point a client at localhost:10102, watch CPU % change.
• A dogfood target for the ha-s7 Home Assistant integration: CPU/temp sensors, lamp switch, brightness slider — all the entity classes on one device.
• Something non-trivial for the diagnostic-sensor latency/count metrics in ha-s7 to exercise.

DB layout

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

DB1.DBD0:REAL    cpu_percent          0..100
DB1.DBD4:REAL    memory_percent       0..100
DB1.DBD8:REAL    disk_read_mbps       megabytes/second
DB1.DBD12:REAL   disk_write_mbps      megabytes/second
DB1.DBD16:REAL   net_rx_mbps          megabytes/second
DB1.DBD20:REAL   net_tx_mbps          megabytes/second
DB1.DBD24:REAL   cpu_temp_c           0 if sensor not available
DB1.DBD28:REAL   fan_rpm              0 if sensor not available
DB1.DBD32:DINT   uptime_seconds
DB1.DBX36.0:BOOL overheating          cpu_temp > 75
DB1.DBX36.1:BOOL high_load            cpu_percent > 80
DB1.DBX36.2:BOOL disk_busy            disk_read+write > 50 MB/s
---
DB2.DBX0.0:BOOL  lamp_on
DB2.DBX0.1:BOOL  alarm_enable
DB2.DBW2:INT     brightness           0..255
DB2.DBW4:INT     setpoint_c
DB2:10:STRING[32] message

Platform notes

• CPU temperature and fan speed report 0 where psutil has no sensor backend (macOS, most containers). Intentional, not a bug.
• psutil lives behind a new demo extra so the core install stays thin.

Test plan

• Unit tests for MetricCollector, _encode_sensors, threshold BOOL derivation, and ControlWatcher. 5 new tests, all pass.
• End-to-end smoke: started the demo server, connected a real snap7.Client, read DB1 → got live values (CPU 34.3%, MEM 55.6%). Wrote DB2 → demo logged [WRITE] lamp_on = ON, brightness = 200.
• Full suite: 1506 passed, 91 skipped (+5 new).
• mypy + ruff + pre-commit clean.
• Skips gracefully if psutil isn't installed (via pytest.importorskip).

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