tests: guard CacheStatus as_str/serde encodings against drift

[cargo-affected-bot]

Found during the nightly rolling survey of src/report.rs.

CacheStatus carries two independent kebab-case encodings that must agree:

• as_str() — the manual mapping, reached via the Display impl for the stderr summary line (cargo-affected: cache=<status> …).
• the serde #[serde(rename_all = "kebab-case")] derive — used for the JSON report.

Nothing enforced that they stay in sync. The only existing test (cache_status_serializes_kebab_case) checked a single variant, and the as_str doc comment was inaccurate — it claimed the string was "used by the JSON serializer" (the serializer uses serde's own derive, not as_str) and that there was "one canonical mapping" (there are two). A future rename of one encoding without the other would drift silently — exactly the failure mode the project's "fail loudly" principle is meant to prevent.

This PR adds cache_status_as_str_matches_serde, which asserts as_str(), Display, and the serde output agree for every variant, and corrects the doc comment to describe the relationship accurately.

No behavior change — this is a test + doc-comment change guarding an existing invariant. cargo clippy --all-targets and the report:: test module are clean locally.

[cargo-affected-bot]

Self-review — the guard is correct and the doc-comment fix is accurate (Display/as_str for the stderr line, serde derive for JSON, verified against summary_line's cache={status}).

One observation inline: the test's variant list is itself a third hand-maintained copy that can drift silently — the exact failure class this PR is guarding against, one level up.

[cargo-affected-bot]

This array is a third hand-maintained enumeration of the variants. as_str's match is exhaustive, so the compiler forces a new variant to be added there — but nothing forces it into this array, so a newly-added variant would silently escape the guard. That's the same silent-drift failure mode the PR is closing, just one level up.

A dependency-free way to make it fail loudly: drop an exhaustive match next to the array so adding a variant breaks compilation until it's listed here too, e.g.

// Compile-time reminder: a new variant must be added to the array above.
fn _exhaustive(s: CacheStatus) {
    match s {
        CacheStatus::HitExact
        | CacheStatus::HitWithDivergence
        | CacheStatus::MissFingerprint
        | CacheStatus::MissNoCoverage
        | CacheStatus::MissNoReachableSha
        | CacheStatus::ForcedAll => {}
    }
}

Not blocking — the current test fully covers the rename-one-encoding case it targets.