feat: extract k-mer entropy computation into new obikentropy crate

Extracts streaming entropy logic and sliding-window frequency tracking from obiskbuilder into a dedicated obikentropy crate. Introduces an EntropyTracker accumulator for O(1) per-base normalized Shannon entropy, replaces inline rolling statistics with delegated state management, and updates workspace dependencies across obikindex, obikpartitionner, and obiskbuilder. Adds criterion benchmarks to validate the refactored pipeline throughput.

src/obikpartitionner/Cargo.toml

@@ -13,6 +13,7 @@ obikrope  = { path = "../obikrope" }
 niffler = "3.0.0"
 remove_dir_all = "0.8"
 obikseq      = { path = "../obikseq" }
+obikentropy  = { path = "../obikentropy" }
 obiskbuilder = { path = "../obiskbuilder" }
 obiskio      = { path = "../obiskio" }
 obidebruinj  = { path = "../obidebruinj" }