Migrate without shims

Converge on the target design instead of carrying compatibility code toward it.

Replacing an internal API

Only when all three hold: nothing outside depends on the API, the change is part of a simplification, and the project can absorb a coordinated breaking change. If any fails, this branch does not apply.

• Migrate every caller and delete the old API in the same change, with no compatibility layer left behind.

A planned migration with declared phase boundaries

• An authorized migration group with declared interior breakage is one verifiable unit. Only its declared breakage is allowed; any undeclared failure stops the work; its boundary checks pass before dependent work proceeds.
• Keep fast, high-signal checks running on the areas being changed throughout.
• At the end of the migration or rewrite, run the full static and runtime verification, not only the checks the last change touched.