feat: add obilayeredmap crate for disk-backed k-mer indexing

Introduces the `obilayeredmap` crate (v0.1.0), implementing an append-only, disk-backed k-mer index using a minimal perfect hash function (MPHF). The module features memory-mapped reads, buffered writes, custom error handling, partition metadata persistence, and comprehensive unit tests. Also adds a reverse complement benchmark for `obikseq` and updates `Cargo.lock` with the new dependencies.

src/obikseq/benches/superkmer.rs

@@ -1,5 +1,7 @@
 use criterion::{BatchSize, BenchmarkId, Criterion, Throughput, criterion_group, criterion_main};
+use obikseq::packed_seq::PackedSeq;
 use obikseq::superkmer::SuperKmer;
+use obikseq::Sequence;
 
 const LENGTHS: &[usize] = &[1, 4, 8, 16, 40, 64, 128, 255, 256];
 
@@ -50,12 +52,12 @@ fn bench_write_ascii(c: &mut Criterion) {
 fn bench_revcomp(c: &mut Criterion) {
     let mut group = c.benchmark_group("revcomp");
     for &len in LENGTHS {
-        let sk = SuperKmer::from_ascii(&make_ascii(len));
+        let seq = PackedSeq::from_ascii(&make_ascii(len));
         group.throughput(Throughput::Bytes(len as u64));
-        group.bench_with_input(BenchmarkId::from_parameter(len), &sk, |b, sk| {
+        group.bench_with_input(BenchmarkId::from_parameter(len), &seq, |b, seq| {
             b.iter_batched(
-                || sk.clone(),
-                |mut s| { std::hint::black_box(&mut s).revcomp(); s },
+                || seq.clone(),
+                |s| { let mut s = std::hint::black_box(s); s.revcomp_inplace(); s },
                 BatchSize::SmallInput,
             );
         });