Add persistent compact integer vector and cache-line-optimized MPHF

Introduce the `obicompactvec` crate, featuring a two-tier, memory-mapped integer vector that uses a primary `u8` array with a sentinel for overflow dispatch and a sparse L1-resident index for fast random access. Implement builder and reader modules with zero-copy serialization and comprehensive test coverage. Update `obilayeredmap` to replace the default hash function with a cache-line-optimized `Mphf`, adding explicit bounds checking and duplicate-slot detection. Add documentation for both modules and update project configuration files accordingly.

mkdocs.yml

@@ -45,6 +45,7 @@ nav:
       - MPHF selection: implementation/mphf.md
       - Unitig evidence encoding: implementation/unitig_evidence.md
       - obilayeredmap crate: implementation/obilayeredmap.md
+      - PersistentCompactIntVec: implementation/persistent_compact_int_vec.md
   - Architecture:
       - Sequences: architecture/sequences/invariant.md