Update circularities docs

[tsmbland]

Description

Update docs to express flexible capacities in terms of tranche counts

Fixes # (issue)

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 90.95%. Comparing base (7cd741d) to head (bb610de).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1531   +/-   ##
=======================================
  Coverage   90.95%   90.95%           
=======================================
  Files          61       61           
  Lines        9050     9050           
  Branches     9050     9050           
=======================================
  Hits         8231     8231           
  Misses        500      500           
  Partials      319      319           

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[Copilot]

🟡 Changes recommended

The updated documentation introduces a unit mismatch and omits key bound behaviour (lower-bound clamping / tranche conversion), making the described tranche bounds inconsistent with the implementation.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Updates the investment model documentation to describe circularity-cycle “flexible capacity” in terms of discrete tranche counts (matching the solver’s integer tranche-variable formulation).

Changes:

• Reframes flexible capacity during cycle balancing from continuous capacity to integer tranche counts.
• Updates the objective-cost explanation to reflect tranche-sized cost scaling.
• Adds a note about missing total-capacity-limit handling.

File summaries

File	Description
docs/model/investment.md	Updates the “Capacity flexibility in circularities” section to describe flexibility via tranche-count bounds and tranche-scaled AFC costs.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Lite

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[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.30.

Benchmark suite	Current: bb610de	Previous: 7cd741d	Ratio
example_run/muse1_default example	431070486.5 ns	296044869 ns	1.46
select_best_assets_sequential/01	9506118.0625 ns	7245930.892045455 ns	1.31
select_best_assets_sequential/05	45810823 ns	34925940.8 ns	1.31
select_best_assets_sequential/10	91473270.5 ns	69812982.5 ns	1.31
select_best_assets_sequential/15	136418646.75 ns	104842185.25 ns	1.30

This comment was automatically generated by workflow using github-action-benchmark.