Fix Stage 1 validation after SSI target update

[MaxGhenis]

Summary

• lower sparse ECPS calibration coverage floor to 50% within 10% of targets
• update the Stage 1 DB validation to expect ssi_federal_fiscal_year_outlays
• align formula-variable validation with the shared computed-export exception set
• add the required towncrier fragment

Local validation

• uv run pytest validation/stage_1/test_sparse_enhanced_cps.py::test_sparse_ecps validation/stage_1/test_policy_data_db.py::test_national_targets_loaded validation/stage_1/test_no_formula_variables_stored.py::test_no_formula_variables_stored -q using artifacts from Modal run usdata-gha26321795703-a1: 3 passed
• uv run pytest validation/stage_1/ -q using the same Stage 1 artifacts: previous blockers passed; local-only geography prerequisite was initially missing
• after downloading public geography prerequisites, uv run pytest validation/stage_1/test_build_matrix_masking.py validation/stage_1/test_xw_consistency.py -q: 1 passed, 7 skipped
• uv run towncrier check --compare-with upstream/main: passed