Branch Status for Out-Of-Service Cases

[lukelowry]

Description

Adds a closed parameter to the Branch model, allowing case files to faithfully import out-of-service transmission lines. Implemented as a RealT mask multiplier on residual contributions, following the d65bd4f IEEEST mask precedent (branch-free residuals for AD).

Closes #370.

Proposed changes

• BranchParameters::closed (bool, default true) new optional param in the Branch device's params block.
• RealT closed_{1.0} member on Branch multiplied into all four evaluateBusResidualNN outputs. When closed = false, residual contributions and Jacobian entries are zero, with no change to AD behavior. This approach, therefore, includes out-of-service branches in the sparsity structure. Long term, this should be optimized, but is out of scope here.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

A few of the larger cases have branches that I must omit when translating to GridKit because we don't have a way to declare branch status. This is a part of a greater effort of mine to have as little friction in case translation as possible, so we can have full parity with PowerWorld cases.