GSYE-158: Add the PV RoI cash-flow and indicator engine

[BigTava]

Summary

• Adds gsy_framework/sim_results/roi, a pure engine for D7.3 equations 5 to 16 with no I/O, covering annualisation, the yearly cash flow and balance, payback, discounted payback, NPV, IRR and LCOE.
• Every suppressed indicator carries a cause, such as never_breaks_even, irr_not_unique or non_positive_benefit, and unusable inputs return no result with a stated reason.
• Supports both inverter replacement treatments, discrete and reserve, and suppresses IRR when the cash flow changes sign more than once.
• Tests check the yearly series and every indicator against reference values in a CSV fixture, plus the guard cases and the balance invariants.
• LCOE is asset-level and ignores the ownership share, zero cash flows do not count as sign changes, and payback uses the first zero crossing.

[codecov]

Codecov Report

❌ Patch coverage is 99.54751% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.83%. Comparing base (f542f9e) to head (596834e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #615      +/-   ##
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+ Coverage   67.61%   68.83%   +1.21%     
==========================================
  Files          88       95       +7     
  Lines        5571     5792     +221     
  Branches     1262     1302      +40     
==========================================
+ Hits         3767     3987     +220     
  Misses       1622     1622              
- Partials      182      183       +1     

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