JsonReporter

Renders a Report as machine-readable JSON for tooling and PR comments (design §7, §9). Pure and dependency-free: the JSON is emitted by hand so the core module stays free of the kotlinx-serialization runtime.

The output carries a top-level schemaVersion (currently "1.0"): additive changes bump the minor, a breaking shape change bumps the major. The shape is pinned by a byte-exact golden test — that golden is the public contract (BCV cannot see JSON output).