Add region key to FlowMap

[tsmbland]

Description

Adds a region field to FlowMap to allow for the possibility of flow regions differing from the region of the asset. This will be necessary for trade as we'll probably want to store trade flows in this map as well. (Or will we... this map needs an asset key, so if we wanted trade processes to exist without assets, then trade flows would need to be stored in a different map. Not sure!)

Fixes #1309

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.70%. Comparing base (51590fa) to head (6723361).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1311      +/-   ##
==========================================
+ Coverage   89.69%   89.70%   +0.01%     
==========================================
  Files          58       58              
  Lines        8470     8481      +11     
  Branches     8470     8481      +11     
==========================================
+ Hits         7597     7608      +11     
  Misses        566      566              
  Partials      307      307              

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[tsmbland]

@alexdewar Could also go further than this and add a region field to ProcessFlow, but depends if we want to use ProcessFlow for trade "processes"

Maybe best to leave this as a draft for now and see how we get on with the rest of it

[Copilot]

Pull request overview

Adds an explicit RegionID dimension to the optimisation FlowMap key so flows can be attributed to a (potentially different) region than the asset’s region, in preparation for cross-region/trade flow handling (Fixes #1309).

Changes:

• Extend FlowMap key from (AssetRef, CommodityID, TimeSliceID) to (AssetRef, CommodityID, RegionID, TimeSliceID).
• Update flow-map construction and downstream consumers (investment net demand update, flow CSV writer, and related tests) to handle the new key shape.
• Adjust parent/child flow copying and map retention logic for the new tuple arity.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/simulation/optimisation.rs	Updates FlowMap type and populates RegionID in flow keys during flow-map creation and parent→child copying.
src/simulation/investment.rs	Uses the RegionID from FlowMap keys when updating the net demand map.
src/output.rs	Updates flow writing/test to destructure the new FlowMap key shape (but currently ignores the region value).

Comments suppressed due to low confidence (1)

src/output.rs:637

• write_flows destructures and then discards region_id (_region_id). If FlowMap now allows flow regions to differ from asset.region_id(), the CSV output will silently lose that information and may misattribute flows when trade is introduced. Consider adding a region_id (or flow_region_id to disambiguate) column to CommodityFlowRow and populating it from the FlowMap key, updating the corresponding CSV test expectations as well.

        for ((asset, commodity_id, _region_id, time_slice), flow) in flow_map {
            let row = CommodityFlowRow {
                milestone_year,
                asset_id: asset.id().unwrap(),
                commodity_id: commodity_id.clone(),
                time_slice: time_slice.clone(),
                flow: *flow,

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