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/obikindex/src/merge.rs

@@ -5,7 +5,7 @@ use std::path::Path;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
 
-use obisys::{MemoryBudget, Reporter, Stage, available_memory_bytes, progress_bar, spinner};
+use obisys::{MemoryBudget, Reporter, Stage, available_memory_bytes, peak_rss_bytes, progress_bar, spinner};
 use rayon::prelude::*;
 use tracing::{debug, info};
 
@@ -200,21 +200,25 @@ impl KmerIndex {
             let worst_id    = order[0];
             let worst_bytes = partition_sizes[worst_id];
 
+            let rss_before_pilot = peak_rss_bytes();
             let worst_g_len = dst_partition
                 .merge_partition(worst_id, &srcs, mode, n_dst_genomes, block_bits, &evidence)
                 .map_err(OKIError::Partition)?;
+            let rss_after_pilot = peak_rss_bytes();
             pb.inc(1);
 
-            let seed_expansion = if worst_bytes > 0 {
-                worst_g_len as u64 * 16 * 1000 / worst_bytes
+            let pilot_rss = rss_after_pilot.saturating_sub(rss_before_pilot);
+            let seed_expansion = if worst_bytes > 0 && pilot_rss > 0 {
+                pilot_rss * 1000 / worst_bytes
             } else {
                 FALLBACK_EXPANSION
             };
 
             info!(
                 "merge_partitions: pilot partition {} — {} unitig bytes → {} new kmers, \
-                 expansion {:.2}×",
+                 RSS delta {}, expansion {:.2}×",
                 worst_id, worst_bytes, worst_g_len,
+                fmt_bytes(pilot_rss),
                 seed_expansion as f64 / 1000.0,
             );