fix(v2): enforce method contracts

[benbrandt]

Summary

• restrict custom v2 APIs to underscore-prefixed extension methods
• add method-safe, direction-safe v2 batch descriptors
• validate every built-in direct and batch response before returning it
• keep local void handlers encoded as an empty object while rejecting peer null results

The existing experimental v2 entrypoint and protocol lane remain unchanged.

Testing

• npm run build
• npm run lint -- --no-cache
• npm run format:check
• npm run docs:ts:verify
• npm test (840 tests)