refactor(obilayeredmap): support generic payload types

Replace the hardcoded `Counts` module with a generic `LayerData` trait, parameterizing `Layer` and `LayeredMap` over arbitrary payload types. This decouples read-path access from build-path logic, enabling both set membership and count-based indexing via `PersistentCompactIntVec`. Adds the `obicompactvec` dependency, implements streaming layer construction, and expands test coverage for persistence and multi-layer resolution.

src/obilayeredmap/Cargo.toml

@@ -4,8 +4,9 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-obikseq      = { path = "../obikseq" }
-obiskio      = { path = "../obiskio" }
+obikseq        = { path = "../obikseq" }
+obiskio        = { path = "../obiskio" }
+obicompactvec  = { path = "../obicompactvec" }
 ptr_hash     = "1.1"
 cacheline-ef = "1.1"
 epserde      = "0.8"