[Ticket]: Extract inline test modules into tests/

### Type

Technical debt

### Summary

Move large inline `#[cfg(test)]` modules into their own files. This is the cheapest, lowest-risk portion of the ratchet.

### Intent

Roughly 840 of `config.rs`'s 1351 lines are its precedence test module. Extracting tests reclaims a large share of the debt with near-zero behaviour risk — the right thing to do first.

### Source of truth (links)

- #162 (parent epic), #173 (the baseline that orders this work)

### Acceptance Criteria

1. Test modules extracted where extraction is clean.
2. Every test still runs — **assert the count did not drop**, do not eyeball it.
3. No test body changes.
4. Tests needing crate-private access are documented as staying inline, with the reason.

### Out of Scope

Behaviour changes of any kind. This is a pure refactor.

### Technical Context

Some tests legitimately need private access and must stay. Say which, and why, rather than making items public to move them.

### Risks

A silently dropped test file is invisible in a green run. Compare test counts before and after, explicitly.

### Test Plan

Test count before equals test count after. State both numbers in the PR.