New NPV metric using SNAS

[tsmbland]

Description

Simplifies the NPV metric to use the SNAS calculation described in #1201. Previously, there was a much more complicated ranking method based on two numbers (total_annualised_surplus and annualised_fixed_cost), now it's a much simpler method involving one number (the SNAS), where higher is better.

In practice, this ends up being extremely similar to LCOX, the only real difference being that SNAS includes the commodity of interest whereas LCOX doesn't. Apart from that, SNAS is just the negative of LCOX, and maximised instead of minimised.

There are almost certainly ways we could tidy up the code, but I figured that's a separate job and @alexdewar may already have ideas about that. We also need to retain some flexibility as future objective metrics may be more fundamentally different.

The docs are also of date. I've deleted the entire NPV section and replaced it with a short section below LCOX that just highlights the differences. The LCOX documentation is outdated, but that was already the case, and I figured that's a separate job.

Fixes the circularity_npv model, so I've re-enabled that.

Fixes #1201

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[tsmbland]

Oh great, we have a model differing between OS's again. Fantastic 😐 False alarm!

[Copilot]

Pull request overview

This PR updates the “NPV” investment appraisal tool to use the SNAS-style metric described in #1201, and refreshes regression fixtures/docs accordingly.

Changes:

• Switch NPV appraisal from the previous profitability-index/TAS hybrid comparison to a single per-activity metric (via snas).
• Re-enable and update regression coverage/fixtures for circularity_npv and update simple_npv expected outputs.
• Simplify the investment-tool documentation by removing the old NPV section and noting the intended relationship to LCOX.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/finance.rs	Introduces snas() helper and removes the old profitability index implementation.
src/simulation/investment/appraisal.rs	Updates NPV appraisal to use snas and adjusts metric structs/tests accordingly.
docs/model/investment.md	Rewrites the NPV tool documentation section.
tests/regression.rs	Re-enables the circularity_npv regression test.
tests/data/simple_npv/commodity_prices.csv	Updates expected outputs for simple_npv regression.
tests/data/simple_npv/commodity_flows.csv	Updates expected outputs for simple_npv regression.
tests/data/simple_npv/assets.csv	Updates expected outputs for simple_npv regression.
tests/data/simple_npv/asset_capacities.csv	Updates expected outputs for simple_npv regression.
tests/data/circularity_npv/commodity_prices.csv	Updates expected outputs for circularity_npv regression.
tests/data/circularity_npv/commodity_flows.csv	Updates expected outputs for circularity_npv regression.
tests/data/circularity_npv/assets.csv	Updates expected outputs for circularity_npv regression.
tests/data/circularity_npv/asset_capacities.csv	Updates expected outputs for circularity_npv regression.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.60%. Comparing base (cb8a34d) to head (05b8d8e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1379      +/-   ##
==========================================
- Coverage   89.68%   89.60%   -0.09%     
==========================================
  Files          58       58              
  Lines        8406     8338      -68     
  Branches     8406     8338      -68     
==========================================
- Hits         7539     7471      -68     
  Misses        550      550              
  Partials      317      317              

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[dc2917]

Just made a comment on the docs, but the code itself looks good - implementation in src/finance.rs looks fine, and it seems reasonable to have snas defined as a function in a separate module, that happens to be the metric used for NPV.

[tsmbland]

@alexdewar Merging now, but you may want to look at when you're back