BUG: Renewable Controller edge case

[lukelowry]

Description

Improves the numerical conditioning of the REGCA and REECB algebraic equations and cleans up their documentation and tests.

Proposed changes

• Replace REGCA's implicit HVRCM constraint with an explicit smooth gain law.
• Remove REECB's ILMAX continuation variable and compute current-circle capacity directly.
• Update initialization and documentation to match the revised equations.
• Replace smoothing-specific answer keys with ideal values and shared smoothing tolerances while retaining Enzyme and dependency-tracking coverage.

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.
• Changelog changes are N/A; this is a focused cleanup of existing model implementations.

Further Comments

This was contributing to some convergence failures when a particularly low-impedance fault is applied.

[lukelowry]

I think there is an inconsistant default tolerance issue that is causing issues across platforms because the only reason I observed this issue is that the CI for #564 is failing, but works on my machine

[nkoukpaizan]

A few minor comments.