Replace simplejson with stdlib json

[aquarion]

The codebase uses simplejson in a few places, but stdlib json is sufficient for modern Python 3.x.

Files to update

Search for simplejson imports and replace with json.

Changes needed

1. Replace import simplejson or import simplejson as json with import json
2. Remove simplejson from pyproject.toml dependencies
3. Test that JSON encoding/decoding still works correctly

Notes

• simplejson was historically used for Python 2 compatibility and performance
• Modern stdlib json handles most use cases
• Reduces external dependencies