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/family_scan.rs

@@ -109,8 +109,8 @@ pub(crate) fn sibling_layer_dirs(index: &KmerIndex) -> OKIResult<Vec<PathBuf>> {
         if !index_dir.exists() {
             continue;
         }
-        let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
-        for l in 0..meta.n_layers {
+        let n_layers = index.partition().n_layers(part)?;
+        for l in 0..n_layers {
             let this_layer_dir = index.partition().layer_dir(part, l);
             let annex_path = this_layer_dir.join(ANNEX_FILE_NAME);
             if !annex_path.exists() {