You are given summary statistics for two independent groups of measurements: the sample mean, the sample standard deviation, and the sample size for each group. You do NOT have access to the raw data points. Write a function pooled_stats(m1, s1, n1, m2, s2, n2) that returns a tuple (combined_mean, combined_sd) where combined_mean is the mean of all observations from both groups treated as one dataset, and combined_sd is the sample standard deviation (denominator N - 1) of that combined dataset. The provided standard deviations are sample standard deviations (denominator n - 1).