Add pysim - Python port of C++SIM

[steveant]

Summary

• Complete Python 3.12+ port of C++SIM using SimPy as the underlying simulation engine
• Preserves C++SIM API while adapting to Python idioms (generator-based coroutines instead of OS threads)
• PRNG produces identical sequences to C++SIM with same seeds
• 77 validation tests pass against C++ expected_output files

What's Included

Core Classes:

• Process, Scheduler, Entity, Semaphore, TriggerQueue
• SimSet linked lists (Head, Link, Linkage)

Random Streams:

• Uniform, Exponential, Normal, Erlang, HyperExponential, Triangular, Draw

Statistics:

• Mean, Variance, Histogram, PrecisionHistogram, SimpleHistogram, Quantile

Documentation:

• Comprehensive README with API reference and migration guide
• CHANGELOG documenting the port
• Three runnable examples (producer_consumer, machine_shop, stats_demo)
• Type hints throughout with py.typed marker

Test Plan

• All 77 validation tests pass (uv run pytest tests/ -v)
• All three examples run successfully
• Package installs cleanly (uv pip install -e .)
• Import verification: python -c "from pysim import Process, Scheduler; print('OK')"