Centralize partition metadata access and add layer introspection APIs

Replaced scattered direct metadata loading with centralized instance methods on `KmerPartition` to guarantee consistent error mapping and legacy recovery. Introduced `StorageKind`, `LayerContent`, and `EvidenceKind` enums alongside lightweight disk-probe methods that inspect file presence without opening heavy data structures. Updated callers across the index, partitioner, and phylo modules to use the new partition API, and added unit tests validating the introspection behavior.

src/obikphylo/src/siblings/build.rs

@@ -8,7 +8,7 @@ use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
 use obilayeredmap::MphfLayer;
-use obilayeredmap::meta::PartitionMeta;
+use obilayeredmap::meta::IndexMode;
 use obisys::progress_bar;
 
 use obikindex::{OKIError, OKIResult};
@@ -97,11 +97,13 @@ impl SiblingAnnexBuildExt for KmerIndex {
                 pb.inc(1);
                 continue;
             }
-            let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
+            let meta = self.partition().partition_meta(part)?;
 
             let mut part_slots: u64 = 0;
             for l in 0..meta.n_layers {
-                part_slots += build_layer_sibling_annex(self, &self.partition().layer_dir(part, l), n_parts, l, &cache)?;
+                part_slots += build_layer_sibling_annex(
+                    self, &self.partition().layer_dir(part, l), &meta.mode, n_parts, l, &cache,
+                )?;
             }
             total_slots += part_slots;
             pb.inc(1);
@@ -120,13 +122,12 @@ impl SiblingAnnexBuildExt for KmerIndex {
 fn build_layer_sibling_annex(
     index: &KmerIndex,
     layer_dir: &Path,
+    mode: &IndexMode,
     n_parts: usize,
     l: usize,
     cache: &Arc<PartitionCache>,
 ) -> OKIResult<u64> {
-    let index_dir = layer_dir.parent().expect("layer_dir has a parent index dir");
-    let meta = PartitionMeta::load(index_dir).map_err(olm_to_ok)?;
-    let mphf = MphfLayer::open(layer_dir, &meta.mode).map_err(olm_to_ok)?;
+    let mphf = MphfLayer::open(layer_dir, mode).map_err(olm_to_ok)?;
     let k = index.kmer_size();
     let n = mphf.n();