Let the compiler hold the invariant a rejected access is not public

[petrovo-as]

Note

I am not a Rust developer. This follow-up to #380 was written with AI assistance and reviewed line by line before opening.

The problem

Rejection in member_visibility.rs carries a Visibility, which has three values. It can only ever hold two of them: judge_branch settles a public member and returns before any rejection is built. The third value is reachable in the type but not in the program, so three match arms exist for a state that cannot arise — including one in build_message that would produce a wrong message had it ever run.

The change

Rejection now carries a two-valued Restriction (Private or Protected). The single conversion point is the public early-return that was already there, so the one public path is kept and the impossible arms are gone.

No behaviour change. It also removes the arms Codecov flagged as uncovered on #380, though that is a side effect rather than the point.

Testing

cargo clippy --all-targets -- -D warnings, cargo fmt --check, and the member_visibility suite pass unchanged. No test was modified — an unchanged suite is the evidence that behaviour is the same.

No changelog entry: internal refactor with nothing observable for users.

Checklist

If applicable:

• I have updated CHANGELOG.md (internal refactor, nothing observable)
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes (behaviour is unchanged; the existing suite passing unmodified is the check)
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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