Edge case gates for IEEET1 (Validation)

[lukelowry]

Description

A few edge-case guards for parameters that caused the unsimplified versions of our examples to fail.

Proposed changes

• Apply a minimum time constant to Tr, Ta, Te, and Tf to prevent singular residual equations.
• Treat Se1 = Se2 = 0 as disabled saturation and validate enabled saturation parameters before deriving the coefficients.
• Document the parameter handling and add a test for zero time constants and disabled saturation.

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; this is a minor correction to an existing model.

Further comments

This preserves the existing fixed-size IEEET1 structure and allows the unsimplified example data to run without case-file changes. A structural treatment of zero time constants can be considered separately and is the preferred approach. I noticed that making the time constant nonzero, rather than zero, dramatically slows the simulation.