Stabilize the public engine API/SPI for v2

[FumingPower3925]

Turn the outcome of the engine-API spike (#254) into the v2 contract: a documented, supported engine interface, with the transplant/internal machinery moved off the public surface. This is what makes retiring std and adding native engines a clean major bump.

[FumingPower3925]

Its first half, moving the transplant and other internal machinery off the public surface, moved into v1.6.0 as part of #443: after the public v1.6.0 release that would need a /v2 import path. This issue keeps the documented, supported engine SPI, which is additive and can land later.