Add CurveFitter with scipy least_squares, configurable loss functions (soft_l1, huber, cauchy), profile likelihood, confidence intervals, and goodness-of-fit metrics. Add CWExtractor for extracting shape factors from beta spectra via Kurie analysis, parametrization fitting, and g_V/g_A extraction. Includes 32 new tests (18 fitter + 14 CW extractor). Bump version to 0.3.0.