json package public API (packages/json/)

The JSON types and methods (JsonValue, JsonObject, JsonArray, JsonParser, JsonPrinter) are consumed by pony-lsp (tools/pony-lsp/, heavily — message framing, settings, diagnostics) and pony-lint (tools/pony-lint/config.pony). Renaming or removing methods, changing the JsonValue union, or dropping an implemented interface can break those tool builds even though the json package's own tests still pass. Serialization calls in particular hide inside .>/.-chains, so a grep for the old name may miss call sites — let the compiler find them. Run: make test-pony-lsp, make test-pony-lint.