Add OPC UA demo drift mode and controls

[smithaldon1]

Summary

Closes #190.

• Adds deterministic gradual drift mode to the local OPC UA demo simulator.
• Adds explicit demo-only StartDrift and ResetDemo OPC UA methods on the State object.
• Keeps the server in normal mode at startup; drift starts only when StartDrift is called.
• Makes repeated StartDrift calls safe by returning already_active without resetting the drift clock.
• Updates fill-weight, nozzle-pressure, and final-fill-weight values once per second after drift starts, compressed for an 8-10 minute demo.
• Documents trigger behavior, expected timing, reset behavior, and the demo-only/no-arbitrary-write boundary.

Test Evidence

• .venv/bin/python -m pytest services/simulator/tests/test_opcua_demo_server.py services/simulator/tests/test_opc_ua_demo_namespace_docs.py - 9 passed
• make test - 204 passed
• make lint - All checks passed
• make typecheck - compileall completed successfully
• make docs - Markdown-only docs target completed
• docker compose -f infra/docker/docker-compose.yml config - passed
• git diff --check - passed
• git diff --cached --check - passed

Docker runtime note:

• docker compose -f infra/docker/docker-compose.yml up --build -d opcua-simulator could not be completed in this environment because the Docker daemon/socket is unavailable: failed to connect to the docker API at unix:///Users/aldonsmith/.docker/run/docker.sock.
• docker version shows the Docker client is installed, but daemon connection fails with the same socket error.
• The local asyncua server/client test verifies normal-mode values, StartDrift, repeated start behavior, drift value changes, state tag changes, and reset behavior outside Docker.

Docs Impact

• Updated services/simulator/README.md with StartDrift, ResetDemo, compressed drift timing, idempotent start behavior, and reset behavior.
• Updated docs/demo/OPC_UA_DEMO_NAMESPACE.md with demo control methods and demo-only control boundaries.
• Updated docs/LEARNING_LOG.md with the learning summary.

Follow-up Work

• Run the Dockerized service on a machine with Docker Desktop/daemon running to verify container startup logs end-to-end.
• Wire a future UI or smoke-test control path to call these explicit demo methods in a separate issue.

[smithaldon1]

Reviewed and approved for merge