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/build.rs

@@ -4,7 +4,6 @@ use std::sync::Arc;
 
 use rayon::prelude::*;
 
-use obicompactvec::{FamilyMask, SiblingAnnexBuilder};
 use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
@@ -17,7 +16,7 @@ use obikindex::KmerIndex;
 
 use super::cache::PartitionCache;
 use super::helpers::{central_base, is_minorant};
-use super::{olm_to_ok, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnexBuilder, ANNEX_FILE_NAME, INDEX_SUBDIR};
 
 // ── obipipeline data types ─────────────────────────────────────────────────