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/obiskbuilder/benches/superkmer_stream.rs

@@ -0,0 +1,58 @@
+//! Throughput of the streaming superkmer pipeline (`RollingStat`'s hot path:
+//! minimizer selection + entropy tracking fused in a single pass).
+//!
+//! Reference point for the `obikentropy` extraction: the entropy bookkeeping
+//! that used to live inline in `RollingStat` was pulled out into a composed
+//! `EntropyTracker`. This benchmark is run before and after that change to
+//! confirm no regression.
+
+use criterion::{Criterion, Throughput, criterion_group, criterion_main};
+use obikrope::Rope;
+use obiskbuilder::SuperKmerIter;
+
+const K: usize = 21;
+const M: usize = 9;
+const LEVEL_MAX: usize = 6;
+const THETA: f64 = 0.7;
+const SEQ_LEN: usize = 200_000;
+
+/// Deterministic pseudo-random ACGT sequence — high enough complexity that
+/// the entropy filter rarely rejects, so the bench stays on the steady-state
+/// path rather than repeatedly resetting.
+fn make_sequence(len: usize) -> Vec<u8> {
+    let mut state: u64 = 0x9E3779B97F4A7C15;
+    (0..len)
+        .map(|_| {
+            state ^= state << 13;
+            state ^= state >> 7;
+            state ^= state << 17;
+            b"ACGT"[(state % 4) as usize]
+        })
+        .collect()
+}
+
+fn make_rope(seq: &[u8]) -> Rope {
+    let mut rope = Rope::new(None);
+    rope.push(seq.to_vec());
+    rope
+}
+
+fn bench_build_superkmers(c: &mut Criterion) {
+    obikseq::set_k(K);
+    obikseq::set_m(M);
+
+    let seq = make_sequence(SEQ_LEN);
+    let rope = make_rope(&seq);
+
+    let mut group = c.benchmark_group("build_superkmers");
+    group.throughput(Throughput::Bytes(SEQ_LEN as u64));
+    group.bench_function("stream", |b| {
+        b.iter(|| {
+            SuperKmerIter::new(std::hint::black_box(&rope), K, LEVEL_MAX, THETA).count()
+        });
+    });
+    group.finish();
+}
+
+criterion_group!(benches, bench_build_superkmers);
+criterion_main!(benches);