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/obisys/Cargo.toml

@@ -4,4 +4,5 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-libc = "0.2"
+libc     = "0.2"
+sysinfo  = "0.33"

src/obisys/src/lib.rs

@@ -2,6 +2,21 @@ use std::fmt;
 use std::time::Instant;
 
 use libc::{RUSAGE_SELF, getrusage, rusage, timeval};
+use sysinfo::System;
+
+// ── Memory query ──────────────────────────────────────────────────────────────
+
+/// Returns the number of bytes available for allocation on this machine.
+///
+/// On macOS, `available_memory()` can return 0 when the memory compressor
+/// inflates the page count; in that case we fall back to half of total memory.
+pub fn available_memory_bytes() -> u64 {
+    let sys = System::new_all();
+    match sys.available_memory() {
+        0 => sys.total_memory() / 2,
+        n => n,
+    }
+}
 
 // ── raw helpers ───────────────────────────────────────────────────────────────