coverage: ast-grep reports mixed carrying a Rust-specific caveat #106

Split out of #105 review — cosmetic, not a correctness bug, deliberately not held against that PR.

What happens

scanner/astgrep.go marks the ast-grep source mixed with the Rust caveat whenever any analyzed file is Rust:

source := analysis.Source{Name: "ast-grep", Status: analysis.SourceAuthoritative}
for _, fileAnalysis := range analyses {
    if DetectLanguage(fileAnalysis.Path) == "rust" {
        source.Status = analysis.SourceMixed
        source.Detail = rustCoverageNote
        break
    }
}

On a two-crate Cargo workspace that produces:

{"name": "ast-grep",       "status": "mixed",         "detail": "Rust macro-generated, string-routed, and #[path] module edges may be unresolved"}
{"name": "cargo-metadata", "status": "authoritative"}
{"name": "rust-cargo",     "status": "mixed",         "detail": "Rust macro-generated, string-routed, and #[path] module edges may be unresolved"}

Why it's wrong

ast-grep's extraction isn't degraded — it found what it found. The caveat (macro-generated, #[path], string-routed edges) describes a Rust resolution gap, which is rust-cargo's concern. Conflating "which tool ran" with "which language has resolution gaps" has two consequences:

1. On a Go + Rust monorepo, the Go analysis is reported as degraded too, even though nothing about the Go extraction is partial.
2. It's the direct cause of coverage: Rust caveat prints twice in the CLI coverage line #107 — the same sentence lands on two sources and prints twice.

Suggested fix

Leave ast-grep as authoritative and let only rust-cargo carry the Rust caveat. The source list then reads as what each tool actually contributed, and the duplicate note disappears at the same time.

A third site does the same thing: scanner/types.go in newDepsProject sets coverage.Sources[0].Detail = rustCoverageNote on the ast-grep source (kept "for the determinism and Rust-coverage tests").

Reproduce

Cargo workspace with two crates where app uses helper from a #[cfg(test)] block, then:

codemap --json --deps <workspace> | jq .coverage