REGCA Implementation

[lukelowry]

Description

Adds the REGCA renewable generator/converter model to PhasorDynamics, including system-model JSON parsing, signal wiring for optional current-command inputs, monitor support, and Enzyme/dependency-tracking Jacobian coverage.

Proposed changes

• Add Converter/REGCA model, data type, CMake
• Wire Regca into ComponentLibrary, SystemModelData, SystemModel, and JSON parsing.
• Add REGCA unit tests for construction, validation, initialization, residuals, bus injection, JSON/system assembly, and Jac consistency

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.

Further comments

Won't add this to any examples until I have REECA since they go together in all cases we have in PowerWorld

[pelesh]

Blocking this PR until Jacobian implementation in PhasorDynamics is refactored to eliminate deprecated Matrix_COO object.