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/cgsc/tests/TestRepo/CGSC.proj/105_CoolingDemand.flow/04_MonthlyEnergyBalance.step/Grombühl_mini_DIN18599_COOLING.log

+======================== Summary ========================
+
+
+
+================ Excluded buildings lists ===============
+

old/cgsc/tests/TestRepo/CGSC.proj/105_CoolingDemand.flow/04_MonthlyEnergyBalance.step/params.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<java version="1.8.0_422" class="java.beans.XMLDecoder">
+ <object class="eu.simstadt.workflowsteps.MonthlyEnergyBalanceStep">
+  <void property="name">
+   <string>Base</string>
+ </void>
+  <void property="calculationMode">
+   <object class="java.lang.Enum" method="valueOf">
+    <class>de.hft.stuttgart.simstadt2.buildingenergymodels.MonthlyBalanceDIN18599$CalculationMode</class>
+    <string>COOLING</string>
+   </object>
+   </void>
+ </object>
+</java>

old/cgsc/tests/TestRepo/CGSC.proj/105_CoolingDemand.flow/params.xml

+<java version="1.8.0_422" class="java.beans.XMLDecoder">
+ <object class="eu.simstadt.workflows.CityGmlWorkflow" id="CityGmlWorkflow0">
+  <void class="de.hftstuttgart.hierarchicworkflow.Workflow" method="getField">
+   <string>selectedStartStepPath</string>
+   <void method="set">
+    <object idref="CityGmlWorkflow0" />
+    <string>00_ImportCityGml.step</string>
+   </void>
+  </void>
+  <void class="de.hftstuttgart.hierarchicworkflow.WorkflowStep" method="getField">
+   <string>startStep</string>
+   <void method="set">
+    <object idref="CityGmlWorkflow0" />
+    <boolean>true</boolean>
+   </void>
+  </void>
+  <void property="name">
+   <string>CGSC_HeatDemand</string>
+  </void>
+  <void property="shortName">
+   <string>HeatDemand</string>
+  </void>
+  <void property="workflowProvider">
+   <object class="de.hftstuttgart.simstadtworkflows.energy.HeatDemandAnalysisWorkflowProvider" />
+  </void>
+ <void property="cityGmlFileNames"><void method="add"><string>Gromb&#252;hl_mini.gml</string></void></void></object>
+</java>
\ No newline at end of file

old/cgsc/tests/TestRepo/CGSC.proj/results/.gitignore

+*.csv