refactor: replace rayon with NUMA-aware PartitionRunner

Replaces `rayon` parallel iteration across index, rebuild, reindex, and select modules with a custom `PartitionRunner`. This introduces NUMA-aware task distribution with CPU pinning and round-robin scheduling, eliminating `Arc`, `Mutex`, and atomic synchronization primitives in favor of a flat, pre-spawned worker architecture. Error handling is simplified via `.map_err()` and the `?` operator, while progress bar updates are decoupled into dedicated callbacks.

src/obicompactvec/src/intmatrix.rs

@@ -1,6 +1,6 @@
 use std::cmp::Ordering;
 use std::fs::{self, File};
-use std::io::{self, Write as _};
+use std::io::{self, BufWriter, Write as _};
 use std::path::{Path, PathBuf};
 
 use memmap2::Mmap;
@@ -354,30 +354,47 @@ impl PackedCompactIntMatrix {
 
 /// Build `counts/matrix.pcmx` from existing `col_*.pciv` files.
 pub fn pack_compact_int_matrix(dir: &Path) -> io::Result<()> {
+    let packed_path = dir.join("matrix.pcmx");
+    if packed_path.exists() {
+        // Matrix complete; remove any leftover column files from a killed cleanup.
+        if let Ok(meta) = MatrixMeta::load(dir) {
+            for c in 0..meta.n_cols { let _ = fs::remove_file(col_path(dir, c)); }
+            let _ = fs::remove_file(dir.join("meta.json"));
+        }
+        return Ok(());
+    }
+
     let meta = MatrixMeta::load(dir)?;
     let n_cols = meta.n_cols;
 
-    let col_files: Vec<Vec<u8>> = (0..n_cols)
-        .map(|c| fs::read(col_path(dir, c)))
+    // Compute offsets from file sizes — no column data loaded into RAM.
+    let col_sizes: Vec<u64> = (0..n_cols)
+        .map(|c| fs::metadata(col_path(dir, c)).map(|m| m.len()))
         .collect::<io::Result<_>>()?;
 
-    let header_size = PCMX_HEADER + n_cols * 8;
+    let header_size = (PCMX_HEADER + n_cols * 8) as u64;
     let mut col_offset = header_size;
     let mut offsets = Vec::with_capacity(n_cols);
-    for data in &col_files {
-        offsets.push(col_offset as u64);
-        col_offset += data.len();
+    for &size in &col_sizes {
+        offsets.push(col_offset);
+        col_offset += size;
     }
 
-    let packed_path = dir.join("matrix.pcmx");
-    let mut file = File::create(&packed_path)?;
-    file.write_all(&PCMX_MAGIC)?;
-    file.write_all(&[0u8; 4])?;
-    file.write_all(&(meta.n as u64).to_le_bytes())?;
-    file.write_all(&(n_cols as u64).to_le_bytes())?;
-    for &off in &offsets { file.write_all(&off.to_le_bytes())?; }
-    for data in &col_files { file.write_all(data)?; }
-    drop(file);
+    // Write to a temp file; rename atomically so a killed process never leaves
+    // a truncated matrix.pcmx that would be mistaken for a complete file.
+    let tmp_path = dir.join("matrix.pcmx.tmp");
+    let mut out = BufWriter::new(File::create(&tmp_path)?);
+    out.write_all(&PCMX_MAGIC)?;
+    out.write_all(&[0u8; 4])?;
+    out.write_all(&(meta.n as u64).to_le_bytes())?;
+    out.write_all(&(n_cols as u64).to_le_bytes())?;
+    for &off in &offsets { out.write_all(&off.to_le_bytes())?; }
+    for c in 0..n_cols {
+        io::copy(&mut File::open(col_path(dir, c))?, &mut out)?;
+    }
+    out.flush()?;
+    drop(out);
+    fs::rename(&tmp_path, &packed_path)?;
 
     for c in 0..n_cols { fs::remove_file(col_path(dir, c))?; }
     fs::remove_file(dir.join("meta.json"))?;