[Epic] Techno-economic reporting scaffold

[andres9403]

Goal

Deliver a minimal techno-economic reporting scaffold: documented, referenced
cost-relevant quantities in SimExec.py, with units annotated per AGENTS.md.
This is the WP4 techno-economic interface - a reporting scaffold, not a costing
library.

Sub-issues

• Document SimExec labor cost assumptions and references #127 - document SimExec labor cost assumptions and references
• Add references for SimExec cost constants #128 - add references for SimExec cost constants
• Audit SimExec documentation and units #129 - audit SimExec documentation and units

These three are follow-ups from the same PR (#125) review, all touch
SimExec.py, and can land in one PR. #128 in particular overlaps #127 and #129.

Acceptance criteria

• Every CAPEX/OPEX/labor constant and correlation in SimExec.py has an original reference or an explicit, labelled assumption
• Units in bracketed form throughout, e.g. [USD/kg], [USD/GJ], [USD/yr], [-]
• NumPy-style docstrings on SimulationExec cost methods, including GetLabor
• The labor-cost basis question in Document SimExec labor cost assumptions and references #127 resolved explicitly (hourly [1/h] vs per-shift [1/shift])
• A minimal reporting interface that returns cost-relevant quantities in documented units
• At least one test covering the reporting path

Scope boundary

Not in scope: a full costing library, capital-cost estimation methods beyond
what already exists, or optimization over cost.

Effort

1-2 FTE-weeks.