Replace public **kwargs estimator constructors with explicit signatures for stable models

[mkumar73]

The current public estimator constructors are too open. Using broad **kwargs makes the API flexible, but it also makes it harder to understand, harder to document, and easier to misuse.

This issue replaces public **kwargs constructors for stable models with explicit constructor signatures. Stable estimators should expose clearly named parameters instead of accepting a large undefined bag of arguments.

This work should focus on the stable model set only. Experimental models can remain more flexible for now.