Add world.json schema validation with JSON Schema

Create a JSON Schema file for world.json that validates structure, required fields, and ID formats. Add a build-time or load-time validation step. This catches content errors earlier than the runtime WorldValidator and gives better error messages for content authors.

Start by locating world.json and the runtime WorldValidator to understand the existing structure, required fields, and ID formats. Define a JSON Schema for those rules, then identify the build-time or load-time entry point where validation should run. Done means malformed world.json content is rejected earlier with clearer errors.

Written by the indexing model from the issue text.