The service is a library with a thin binary over it, which is what makes
the move possible: an integration test is a separate crate and reaches
only what the library makes public, so the modules now live in
`src/lib.rs` and `main.rs` is the entry point. `src/` holds no test
code, which is the shape `apsis-rs` already has.
Eleven items are internal to a module and a test reaches them anyway.
Those stay `pub(crate)` where they are defined and are offered through
`src/testkit.rs`, which is the one place a caller is expected to look for
them. A wrapper is also the only way to offer a private thing outward:
Rust does not let visibility be widened twice, so a `pub(crate)` item
cannot be re-exported as `pub`. That keeps the seams in one file instead
of scattering them through the modules under test.
The 146 module tests and the 23 end to end tests all still run, which is
the number this move had to preserve: the 146 are spread over eighteen
files in `tests/`, one per module that had them.
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