Epic: Expose 'Reasons' for Flag evaluations

This epic covers the functionality to expose the exact reason why a flag was evaluated to have a given enabled state and value. The possible reasons should be:

1. Environment Default
2. Segment Override
3. Identity Override

No files or tests are named. Start by locating the flag-evaluation entry point and trace where the enabled state and value are produced, then identify the surfaces that expose them. Done means evaluations consistently report one of the three listed reasons: Environment Default, Segment Override, or Identity Override.

Written by the indexing model from the issue text.