Improve rent uprating by tenure

[MaxGhenis]

Summary

• uprate private rents directly from ONS private rental price growth by region instead of backing them out from aggregate OBR rent
• update social rent growth to observed RSH stock-average growth through 2025, then the England policy cap forecast
• add a regression test covering the tenure-specific 2025 rent uprating path

Validation

• uv run pytest policyengine_uk/tests/test_rent_uprating.py -q
• uv run pytest policyengine_uk/tests/test_fiscal_year_parameters.py -q

Notes

• private rent forecasts after observed ONS data use the March 2026 OBR aggregate rent forecast as the documented fallback
• uv.lock is intentionally excluded from this PR