clone typing stabilization: frontend/C parity, tests, and wasm artifacts

- implement clone return-type specialization after method resolution across JS/C frontend and IR paths

- enforce/validate override return covariance and remove legacy Self special diagnostics paths

- add clone-focused conformance tests (deep inheritance, overrides, non-generalization) and update manifest/spec refs

- regenerate wasm/vsix artifacts to keep runtime parity with updated C frontend

Improving spec