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/obilayeredmap/src/map.rs

@@ -104,3 +104,4 @@ fn layer_dir(root: &Path, i: usize) -> PathBuf {
 #[cfg(test)]
 #[path = "tests/map.rs"]
 mod tests;
+