The assembled configuration surface.
The application assembles the surface, because it is the only layer that knows every half of it: this package supplies the runtime-neutral defaults, an adapter package supplies the platform-backed ones, and the application supplies the overrides. Nothing here cares which half a key came from, so supporting another platform means editing the application rather than this file.
A second call replaces the surface rather than merging into it. That keeps a single call unambiguous, and gives a test a clean way to install a fixture.
Registers the configuration surface this process runs on.
Call once at startup, before anything resolves a setting. Everything downstream reads it back through resolve.