Summary

• Adds a Response::json(mixed \$data, int \$encodingOptions = 0): void helper that sets Content-Type: application/json and emits json_encode(\$data, \$encodingOptions | JSON_THROW_ON_ERROR).
• Unlike generateRest* helpers, it does not exit and does not wrap in a meta envelope — use it when you need to send a raw JSON body.
• Throws \JsonException on unencodable values (JSON_THROW_ON_ERROR is always applied).
• Documents the new method in docs/core-features/rest-api.md and adds a line to Changelog.md under v1.2.0.

Test plan

• docs/core-features/rest-api.md renders correctly in the docs site.
• A controller calling \$res->json(['ok' => true]) returns {"ok":true} with Content-Type: application/json and status 200.
• Passing a non-encodable value (e.g. a resource or a circular reference) raises JsonException.

Merge order

This PR targets feature/asset-proxy. Merge this first, then merge PR #112 into main.