Off-nominal branch tap and phase support

[lukelowry]

Description

Adds off-nominal tap magnitude and phase shift support to the PhasorDynamics Branch model so branch devices can represent line and transformer-style branches through the same component.

Proposed changes

• Added tap and phase branch parameters with defaults of 1.0 and 0.0.
• Updated branch admittance/residual calculations for off-nominal tap and phase shift behavior.
• Added parser coverage, branch residual/Jacobian unit tests, and documentation for the new parameters.

Checklist

• All tests pass. (except existing PE test)
• 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.

Further comments

None