feat: implement persistent layered index and chunked binary format

Introduce the `obilayeredmap` specification and persistent MPHF-based index architecture for incremental multi-dataset indexing. Implement chunked binary serialization with a fixed `u8` k-mer count limit (256) and overlapping super-kmer segments. Add memory-mapped I/O and a companion `.idx` index file for allocation-free, O(1) unitig access. Update MkDocs navigation, enhance the k-mer comparison script, and add comprehensive tests for serialization, partitioning, and file I/O pipelines.

src/Cargo.lock

@@ -1615,7 +1615,10 @@ version = "0.1.0"
 dependencies = [
  "memmap2",
  "niffler 3.0.0",
+ "obikrope",
  "obikseq",
+ "obiread",
+ "obiskbuilder",
  "obiskio",
  "ph",
  "rayon",
@@ -1623,6 +1626,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sysinfo 0.33.1",
+ "tempfile",
  "tracing",
 ]
 
@@ -1679,6 +1683,7 @@ name = "obiskio"
 version = "0.1.0"
 dependencies = [
  "lru",
+ "memmap2",
  "niffler 3.0.0",
  "obikseq",
  "rustix",