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.

Cargo.toml

@@ -16,7 +16,7 @@ categories = ["algorithm", "science", "data-structures"]
 readme = "README.md"
 
 [dependencies]
-rand = "0.10"
+fastrand = "2.3"
 thiserror = "2"
 parking_lot = "0.12"
 tokio = { version = "1", features = ["sync", "rt-multi-thread"], optional = true }