Split platformNative & languageNative project up into some smaller pieces

This project is getting a bit large, which means the tests are taking a while to run, and mixes a few things together, which means tests are being invalidated on changes that don't affect them.

Possibly split by language or by model (e.g. software model stuff in one project, current model stuff in another) or both.

Review the platformNative and languageNative projects and how their tests are currently grouped. Compare possible boundaries by language, by model, or both, and define the split so unrelated changes no longer invalidate or require the full test suite.

Written by the indexing model from the issue text.