fix: replace hand-rolled quickselect with select_nth_unstable_by

[agene0001]

select_inplace implements the Numerical Recipes quickselect by hand. With NaN in the slice, its partition loop walks past the end of the array — the crash reported in #163 (a segfault under the pre-0.13 unsafe version, an index-out-of-bounds panic today). Reproducer that panics on main:

let mut v: Vec<f64> = (0..1000).map(|i| (i as f64 * 0.7).sin()).collect();
for i in (0..1000).step_by(7) { v[i] = f64::NAN; }
Data::new(v).quantile(0.5); // panicked at src/statistics/slice_statistics.rs:106:28

Replacing it with <[T]>::select_nth_unstable_by(rank, f64::total_cmp) fixes it three ways:

• NaN gets a defined position in a total order, so the search stays in bounds (previous behaviour with NaN was arbitrary).
• Worst case is O(n) (introselect) instead of the NR quickselect's quadratic worst case.
• ~4.4x faster on 1e6 uniform elements (4.49 ms → 1.02 ms including the clone), ~1.7x at 1e3.

The regression test covers NaN scattered through a large slice, all-NaN input, and NaN at the exact positions the old partition tripped on. Existing order-statistic/median/quantile tests (including test_median_robust_on_infinities) pass unchanged.

Closes #163

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