Port SimStadt library code from old/ into src/simstadtpy/

Add results.py (KPI, SimStadtResults ABC + HeatDemand/PV/GreenWater subclasses),
runner.py (workflow execution), workflows.py (high-level simulation helpers),
and utils.py. Add pytest-based test suite (46 tests, all passing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

old/sektorsim/run_simstadt_from_python/Repository/.gitignore

+*
+!.gitignore