perf: enable zero-allocation queries and memory-mapped indexes

Introduce zero-allocation row extraction and query result buffers across `obicompactvec` and `obikpartitionner` to eliminate per-kmer heap allocations. Replace in-memory MPHF deserialization with memory-mapped, zero-copy views to reduce runtime memory footprint. Add configurable I/O chunking, a RAM-aware `--chunk-size` parameter, and system memory monitoring via the new `sysinfo` dependency. Re-export `PreloadedIndex` for external consumers.

src/obiread/src/chunk.rs

@@ -153,11 +153,23 @@ pub fn read_fastq_chunks(
 /// Returns an error if the format cannot be identified as `text/fasta` or `text/fastq`.
 pub fn read_sequence_chunks(
     path: &str,
+) -> io::Result<SeqChunkIter<MimeTypeGuesser<Box<dyn Read + Send>>>> {
+    read_sequence_chunks_sized(path, DEFAULT_BLOCK_SIZE)
+}
+
+/// Same as [`read_sequence_chunks`] but with an explicit I/O block size.
+///
+/// Larger values amortise per-partition open/close overhead across more superkmers.
+pub fn read_sequence_chunks_sized(
+    path: &str,
+    block_size: usize,
 ) -> io::Result<SeqChunkIter<MimeTypeGuesser<Box<dyn Read + Send>>>> {
     let input = match xopen(path) {
         Ok(mut i) => match i.mime_type() {
-            Some("text/fasta") => fasta_chunks(i),
-            Some("text/fastq") => fastq_chunks(i),
+            Some("text/fasta") => SeqChunkIter::new(i, block_size,
+                fasta::end_of_last_fasta_entry, Some("text/fasta")),
+            Some("text/fastq") => SeqChunkIter::new(i, block_size,
+                fastq::end_of_last_fastq_entry, Some("text/fastq")),
             _ => {
                 return Err(io::Error::new(
                     io::ErrorKind::InvalidData,

src/obiread/src/lib.rs

@@ -18,7 +18,7 @@ pub mod xopen;
 
 pub use chunk::{
     SeqChunkIter, fasta_chunks, fastq_chunks, read_fasta_chunks, read_fastq_chunks,
-    read_sequence_chunks,
+    read_sequence_chunks, read_sequence_chunks_sized,
 };
 pub use mimetype::MimeTypeGuesser;
 pub use normalize::{normalize_fasta_chunk, normalize_fastq_chunk, normalize_sequence_chunk};