feat: dynamically compute seed expansion based on RSS

Introduce a `peak_rss_bytes()` utility for accurate per-phase RAM measurement. Replace the genome-length heuristic with a dynamic seed expansion ratio based on actual RSS delta. Explicitly drop the `GraphDeBruijn` instance before MPHF construction to prevent resource contention and ensure proper memory management.

src/obisys/src/lib.rs

@@ -119,6 +119,12 @@ use sysinfo::System;
 ///
 /// 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.
+/// Returns the process peak RSS (high-water mark since process start).
+/// Monotonically increasing — use delta before/after a phase to measure its RAM cost.
+pub fn peak_rss_bytes() -> u64 {
+    rss_to_bytes(&get_rusage())
+}
+
 pub fn available_memory_bytes() -> u64 {
     let sys = System::new_all();
     let host_avail = match sys.available_memory() {