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/obilayeredmap/src/mphf_layer.rs

@@ -17,8 +17,13 @@ pub(crate) const UNITIGS_FILE:     &str = "unitigs.bin";
 pub(crate) const EVIDENCE_FILE:    &str = "evidence.bin";
 pub(crate) const FINGERPRINT_FILE: &str = "fingerprint.bin";
 
+/// Owned MPHF — used only at build time (construction + store).
 pub(crate) type Mphf = PtrHash<u64, CubicEps, CachelineEfVec<Vec<CachelineEf>>, Xx64, Vec<u8>>;
 
+/// Zero-copy MPHF for querying — ε-deserialized view into a memory-mapped file.
+/// `MemCase` owns the mmap backing; `'static` is sound because MemCase pins the memory.
+type MphfEps = PtrHash<u64, CubicEps, CachelineEfVec<&'static [CachelineEf]>, Xx64, &'static [u8]>;
+
 // ── LayerEvidence ─────────────────────────────────────────────────────────────
 
 enum LayerEvidence {
@@ -36,7 +41,7 @@ enum LayerEvidence {
 /// - [`find_strict`](Self::find_strict) — always exact; O(1) on Exact/Hybrid layers,
 ///   O(n) sequential scan on Approx layers.
 pub struct MphfLayer {
-    mphf: Mphf,
+    mphf: MemCase<MphfEps>,
     ev:   LayerEvidence,
     n:    usize,
 }
@@ -45,7 +50,7 @@ impl MphfLayer {
     /// Open a layer using the index-level `mode` determined at `LayeredMap` open time.
     /// No per-layer metadata file is read.
     pub fn open(dir: &Path, mode: &IndexMode) -> OLMResult<Self> {
-        let mphf: Mphf = Mphf::load_full(&dir.join(MPHF_FILE))
+        let mphf: MemCase<MphfEps> = Mphf::mmap(&dir.join(MPHF_FILE), Flags::empty())
             .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
         let (ev, n) = match mode {
             IndexMode::Exact => {
@@ -137,11 +142,11 @@ impl MphfLayer {
 ///
 /// Use this when the caller guarantees that all queried kmers are in the MPHF
 /// domain (e.g. when iterating the source's own unitigs during merge).
-pub struct MphfOnly(Mphf);
+pub struct MphfOnly(MemCase<MphfEps>);
 
 impl MphfOnly {
     pub fn open(dir: &Path) -> OLMResult<Self> {
-        let mphf: Mphf = Mphf::load_full(&dir.join(MPHF_FILE))
+        let mphf: MemCase<MphfEps> = Mphf::mmap(&dir.join(MPHF_FILE), Flags::empty())
             .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
         Ok(Self(mphf))
     }