Add a fallback price to investment appraisal

[tsmbland]

Description

Modifying the appraisal mini-optimisation to encourage assets to dispatch, as discussed with @ahawkes. I've updated the documentation to fully describe the new approach and reasoning, so I'll refer you to that rather than describing the approach here.

Fixes #1368
Fixes #1384
Makes #1347 much less likely, but we should still come up with a fix for that (even just clarifying the error message)

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

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.47%. Comparing base (b363e64) to head (a4b37d2).

Files with missing lines	Patch %	Lines
src/simulation/prices.rs	91.17%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           investment_docs    #1420      +/-   ##
===================================================
+ Coverage            86.80%   89.47%   +2.66%     
===================================================
  Files                   59       59              
  Lines                 8384     8419      +35     
  Branches              8384     8419      +35     
===================================================
+ Hits                  7278     7533     +255     
+ Misses                 790      564     -226     
- Partials               316      322       +6     

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[Copilot]

Pull request overview

This PR introduces a fallback price concept for the investment-appraisal mini-dispatch optimisation, intended to encourage candidate assets to dispatch when shadow prices alone would lead to zero/near-zero activity (addressing scenarios described in #1384/#1347 and aiming to fix #1368). It adds a configurable fallback_price_strategy, computes fallback prices alongside existing market/shadow prices, and updates appraisal coefficients and regression fixtures accordingly.

Changes:

• Add fallback_price_strategy to model.toml parameters + JSON schema, and document the new mini-dispatch objective formulation.
• Extend price calculation to produce a third price set (Prices::fallback) derived via the configured strategy.
• Update investment appraisal activity coefficients and refresh regression expected outputs across examples.

Reviewed changes

Copilot reviewed 60 out of 62 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/regression.rs	Regression test definitions updated (but circularity tests still not re-enabled).
tests/data/two_regions/commodity_prices.csv	Updated expected prices for two-regions regression outputs.
tests/data/two_regions/commodity_flows.csv	Updated expected flows for two-regions regression outputs.
tests/data/two_regions/asset_capacities.csv	Updated expected capacities for two-regions regression outputs.
tests/data/two_outputs/assets.csv	Updated expected asset list for two-outputs regression outputs.
tests/data/two_outputs/asset_capacities.csv	Updated expected capacities for two-outputs regression outputs.
tests/data/simple/debug_solver.csv	Updated expected solver debug objective values for simple example.
tests/data/simple/commodity_prices.csv	Updated expected prices for simple regression outputs.
tests/data/simple/commodity_flows.csv	Updated expected flows for simple regression outputs.
tests/data/simple/assets.csv	Updated expected commissioned assets for simple regression outputs.
tests/data/simple/asset_capacities.csv	Updated expected capacities for simple regression outputs.
tests/data/simple_shadow/commodity_prices.csv	Updated expected prices for simple_shadow patched regression outputs.
tests/data/simple_shadow/commodity_flows.csv	Updated expected flows for simple_shadow patched regression outputs.
tests/data/simple_shadow/assets.csv	Updated expected commissioned assets for simple_shadow patched regression outputs.
tests/data/simple_shadow/asset_capacities.csv	Updated expected capacities for simple_shadow patched regression outputs.
tests/data/simple_npv/commodity_prices.csv	Updated expected prices for simple_npv patched regression outputs.
tests/data/simple_npv/assets.csv	Updated expected commissioned assets for simple_npv patched regression outputs.
tests/data/simple_npv/asset_capacities.csv	Updated expected capacities for simple_npv patched regression outputs.
tests/data/simple_marginal/commodity_prices.csv	Updated expected prices for simple_marginal patched regression outputs.
tests/data/simple_marginal/commodity_flows.csv	Updated expected flows for simple_marginal patched regression outputs.
tests/data/simple_marginal/assets.csv	Updated expected commissioned assets for simple_marginal patched regression outputs.
tests/data/simple_marginal/asset_capacities.csv	Updated expected capacities for simple_marginal patched regression outputs.
tests/data/simple_marginal_average/commodity_prices.csv	Updated expected prices for simple_marginal_average patched regression outputs.
tests/data/simple_marginal_average/commodity_flows.csv	Updated expected flows for simple_marginal_average patched regression outputs.
tests/data/simple_marginal_average/assets.csv	Updated expected commissioned assets for simple_marginal_average patched regression outputs.
tests/data/simple_marginal_average/asset_capacities.csv	Updated expected capacities for simple_marginal_average patched regression outputs.
tests/data/simple_ironing_out/commodity_prices.csv	Updated expected prices for simple_ironing_out patched regression outputs.
tests/data/simple_ironing_out/assets.csv	Updated expected commissioned assets for simple_ironing_out patched regression outputs.
tests/data/simple_ironing_out/asset_capacities.csv	Updated expected capacities for simple_ironing_out patched regression outputs.
tests/data/simple_full/commodity_prices.csv	Updated expected prices for simple_full patched regression outputs.
tests/data/simple_full/commodity_flows.csv	Updated expected flows for simple_full patched regression outputs.
tests/data/simple_full/assets.csv	Updated expected commissioned assets for simple_full patched regression outputs.
tests/data/simple_full/asset_capacities.csv	Updated expected capacities for simple_full patched regression outputs.
tests/data/simple_divisible/commodity_prices.csv	Updated expected prices for simple_divisible patched regression outputs.
tests/data/simple_divisible/asset_capacities.csv	Updated expected capacities for simple_divisible patched regression outputs.
tests/data/missing_commodity/commodity_prices.csv	Updated expected prices for missing_commodity regression outputs.
tests/data/missing_commodity/assets.csv	Updated expected commissioned assets for missing_commodity regression outputs.
tests/data/missing_commodity/asset_capacities.csv	Updated expected capacities for missing_commodity regression outputs.
tests/data/circularity/commodity_prices.csv	Added/updated expected prices for circularity example at 2040.
tests/data/circularity/assets.csv	Added/updated expected commissioned assets for circularity example at 2040.
tests/data/circularity/asset_capacities.csv	Added/updated expected capacities for circularity example at 2040.
tests/data/circularity_npv/commodity_prices.csv	Added/updated expected prices for circularity_npv patched example at 2040.
tests/data/circularity_npv/assets.csv	Added/updated expected commissioned assets for circularity_npv patched example at 2040.
tests/data/circularity_npv/asset_capacities.csv	Added/updated expected capacities for circularity_npv patched example at 2040.
src/simulation/prices.rs	Add Prices::fallback and compute fallback prices using a strategy override.
src/simulation/investment/appraisal/coefficients.rs	Incorporate fallback prices into mini-dispatch activity coefficients.
src/model/parameters.rs	Add fallback_price_strategy parameter with default value.
src/commodity.rs	Make PricingStrategy Copy to simplify passing strategy overrides.
schemas/input/model.yaml	Document and validate fallback_price_strategy in the input schema.
docs/model/investment.md	Document fallback prices and the updated mini-dispatch coefficient formulation.

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