Add FRS proxy variables for legacy ESA/JSA modelling

[MaxGhenis]

Summary

• add explicit person-level FRS proxy variables for legacy JSA claimant state and ESA health/support-group approximation
• derive the age cutoff from PolicyEngine's state_pension_age instead of hard-coding 66
• add focused tests covering thresholds, negative cases, helper wiring, and the create_frs hook

Notes

• this PR is intentionally limited to dataset-side proxy groundwork
• it does not add ESA/JSA claimant-count calibration targets yet; the current target layer is still spend-oriented and should be a separate follow-up