Move sibling annex implementation to obikphylo siblings module

Relocates `SiblingAnnex`, `FamilyMask`, and `SiblingAnnexBuilder` from `obicompactvec` to the local `obikphylo::siblings` module. Replaces the previous implementation with a memory-mapped version using `memmap2`, featuring a 2-byte-per-slot layout, explicit bitfield manipulation, and support for concurrent atomic writes. Updates all sibling module imports to local paths and adds the `memmap2` dependency to `obikphylo`.

src/obikphylo/src/siblings/family_scan.rs

@@ -53,7 +53,7 @@ use std::sync::atomic::{AtomicU8, Ordering};
 
 use rayon::prelude::*;
 
-use obicompactvec::{FamilyMask, PersistentBitMatrix, PersistentCompactIntMatrix, SiblingAnnex};
+use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obilayeredmap::Layer;
 use obilayeredmap::meta::PartitionMeta;
@@ -65,7 +65,7 @@ use obikindex::KmerIndex;
 use super::cache::{Mat, PartitionCache};
 use super::helpers::central_base;
 use super::iter::SiblingEntry;
-use super::{olm_to_ok, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnex, ANNEX_FILE_NAME, INDEX_SUBDIR};
 
 /// Families per batch — see the module docs for the memory-vs-per-partition-
 /// density trade-off this picks a point on. At ~90 genomes and a few