HYGOV Documentation + Implementation

[lukelowry]

Description

Implementation and documentation for the HYGOV PhasorDynamics governor model.

Proposed changes

• Added HYGOV model implementation, documentation, and diagram.
• Added HYGOV JSON parsing and SystemModel assembly support.
• Added focused PhasorDynamicsGovernorHygovTest unit coverage.
• Included source-default curve behavior for zeroed source curve data.
• Updated governor registration, input-format docs, and changelog.

Checklist

• All tests pass (except known PE failure)
• 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