fix: fill WRMSSE levels 1-5, regenerate parity fixtures (5k rows), recompute SHA256SUMS (part 2)

fix: fill WRMSSE levels 1-5, regenerate parity fixtures (5k rows), recompute SHA256SUMS

Add final end-to-end explanation document

Add statistical models comparison report

Update SHA256SUMS with M5_EXPLAINED.md

Add student-friendly explanation: 10-year-old explanation with college-level technical details

Update SHA256SUMS with TECHNICAL_REPORT.md

Add comprehensive technical report (TECHNICAL_REPORT.md)

Export: 40 per-store boosters, WRMSSE=145.56

initial commit