Extract the stage loop into openTEPES_ProblemSolvingStageIter

[erikfilias]

Changes

Move the per-stage solve loop out of openTEPES_run (in openTEPES.py) into a new module openTEPES_ProblemSolvingStageIter.py, exposing it as a function StageIterativeSolving.

openTEPES.py still builds the objective and the investment constraints, sets First_st / Last_st and the empty pDuals, and then calls the new driver. The driver holds the (period, scenario, stage) loop — which activates one stage's load levels, builds that stage's operation constraints, and calls ProblemSolving — plus the post-loop work that rebuilds the full stage and load-level sets, reactivates every constraint, and restores the scenario probabilities.

Why

This continues the solver-layer split that already produced …Persistent.py, …Tuning.py and …DualExtraction.py. The stage loop was the last large piece still inlined in openTEPES_run. Pulling it out gives the later re-solve and decomposition drivers a single place to call the stage loop from, and makes openTEPES_run short enough to read at a glance.

Also in this PR

• tests/test_direct_run.py — runs every openTEPES_*.py module as a script (via runpy, with name == "main") to check that each one still imports cleanly that way, i.e. the try/except ImportError relative-import guard's fallback works.
• Architecture diagram (doc/img/openTEPES_architecture.svg and the rendered .png) now shows openTEPES_ProblemSolvingStageIter.py as an implemented (green) box in the solver layer; resolve.py stays white as the one solver module still planned.