refactor: replace rand 0.10 with fastrand 2.3

fastrand is smaller, faster, and has no dependencies. Add rng_util
helper for f64 range generation since fastrand lacks a built-in
equivalent. Migrate all samplers, KDE modules, and fANOVA to use
fastrand's concrete Rng type instead of rand's trait-based generics.

src/rng_util.rs

@@ -0,0 +1,5 @@
+/// Generate a random `f64` in the range `[low, high)`.
+#[inline]
+pub(crate) fn f64_range(rng: &mut fastrand::Rng, low: f64, high: f64) -> f64 {
+    low + rng.f64() * (high - low)
+}