Use calibration log for sparse validation

[MaxGhenis]

Summary

• Replace the expensive sparse enhanced CPS validation matrix rebuild with a check of final-epoch calibration diagnostics already written to calibration_log.csv
• Keep the same >60% within 10% release gate, including absolute-error-scaled target tolerances
• Add a changelog fragment

Tests

• uv run python -m pytest validation/stage_1/test_sparse_enhanced_cps.py::test_sparse_ecps -q
• uv run ruff format --check validation/stage_1/test_sparse_enhanced_cps.py
• git diff --check