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/obikpartitionner/src/merge_layer.rs

@@ -223,12 +223,14 @@ impl KmerPartition {
                 uw.write(&unitig)
             })?;
             uw.close()?;
+            let n = g.len();
+            drop(g); // release GraphDeBruijn before MPHF build
             Layer::<()>::build(&new_layer_dir, block_bits, evidence).map_err(olm_to_sk)?;
-            g.len()
+            n
         } else {
+            drop(g);
             0
         };
-        drop(g);
 
         let new_mphf = if any_new {
             Some(MphfOnly::open(&new_layer_dir).map_err(olm_to_sk)?)