AsyncAPI validator and viewer

It checks the AsyncAPI 2.x and 3.x document shapes: the version field, info, servers, channels, operations, and messages, and it resolves every local $ref (with cycle detection, since a ref loop hangs naive resolvers). It does not run the full official AsyncAPI parser surface: no remote or file reference fetching, no JSON Schema validation of payloads, no binding deep checks, no example validation, and no security requirement semantics beyond checking the scheme exists. Findings say what they saw, not what the runtime will do.