feat: introduce column-major matrix storage and migrate layered map

Introduces `PersistentBitMatrix` and `PersistentCompactIntMatrix` to replace single-file vector storage with a column-major, directory-based layout. Each column is persisted as an individual file alongside a lightweight `meta.json` for dimension tracking. Migrates `obilayeredmap` to use these multi-column structures, updating Rust APIs, query return types, and build signatures. Includes comprehensive documentation, unit and integration tests for persistence and accessors, and refactors distance calculation helpers.

src/obicompactvec/src/intmatrix.rs

@@ -0,0 +1,58 @@
+use std::{fs, io, path::{Path, PathBuf}};
+
+use crate::builder::PersistentCompactIntVecBuilder;
+use crate::meta::MatrixMeta;
+use crate::reader::PersistentCompactIntVec;
+
+fn col_path(dir: &Path, col: usize) -> PathBuf {
+    dir.join(format!("col_{col:06}.pciv"))
+}
+
+pub struct PersistentCompactIntMatrix {
+    cols: Vec<PersistentCompactIntVec>,
+    n:    usize,
+}
+
+impl PersistentCompactIntMatrix {
+    pub fn open(dir: &Path) -> io::Result<Self> {
+        let meta = MatrixMeta::load(dir)?;
+        let cols = (0..meta.n_cols)
+            .map(|c| PersistentCompactIntVec::open(&col_path(dir, c)))
+            .collect::<io::Result<Vec<_>>>()?;
+        Ok(Self { cols, n: meta.n })
+    }
+
+    pub fn n(&self)      -> usize { self.n }
+    pub fn n_cols(&self) -> usize { self.cols.len() }
+    pub fn col(&self, c: usize) -> &PersistentCompactIntVec { &self.cols[c] }
+
+    pub fn row(&self, slot: usize) -> Box<[u32]> {
+        self.cols.iter().map(|c| c.get(slot)).collect()
+    }
+}
+
+pub struct PersistentCompactIntMatrixBuilder {
+    dir:    PathBuf,
+    n:      usize,
+    n_cols: usize,
+}
+
+impl PersistentCompactIntMatrixBuilder {
+    pub fn new(n: usize, dir: &Path) -> io::Result<Self> {
+        fs::create_dir_all(dir)?;
+        Ok(Self { dir: dir.to_path_buf(), n, n_cols: 0 })
+    }
+
+    pub fn n(&self)      -> usize { self.n }
+    pub fn n_cols(&self) -> usize { self.n_cols }
+
+    pub fn add_col(&mut self) -> io::Result<PersistentCompactIntVecBuilder> {
+        let path = col_path(&self.dir, self.n_cols);
+        self.n_cols += 1;
+        PersistentCompactIntVecBuilder::new(self.n, &path)
+    }
+
+    pub fn close(self) -> io::Result<()> {
+        MatrixMeta { n: self.n, n_cols: self.n_cols }.save(&self.dir)
+    }
+}