Fit y = intercept + slope*x to 2–64 declared pairs using positive weights and exact rational normal equations. Returns residuals and weighted SSE rounded to 12 decimal places, plus finite float64 RMSE. Rejects degenerate X values; weights are not verified uncertainties. Not field certification. New modular adapter: retained result recovery for 24 hours, minimal receipt metadata for 90 days. Existing live route unchanged.

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