IEEEST Stabilizer Improvements/Corrections

[lukelowry]

Description

Fixes and cleans up the IEEEST stabilizer implementation and documentation.

This updates the older initial implementation style so that reduced-order notch filters and zero denominator time constants are handled consistently

Proposed changes

• Replace the branchy/safe-inverse notch logic with a fixed DAE form using differential tags.
• Initialize IEEEST from the attached input signal instead of forcing all states to zero.
• Document the implemented equations, initialization, derived parameters, and unsupported compatibility parameters.
• Tests for initialization, residuals, invalid wiring, invalid notch parameters, zero time constants, and Jacobian consistency.

Checklist

• All tests pass for the focused PhasorDynamics coverage.
• 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 N/A

Further comments

This is cleanup/correction work for the existing IEEEST model, not a new model

[lukelowry]

@pelesh this fix is also ready to be merged I forgot to make a PR for this one.