refactor: Enforce Rayon parallelism and fix merge_layer concurrency

Updated memory guidelines and feedback docs to explicitly classify intra-partition phases as parallel, correcting prior assumptions of sequential execution. Refactored merge_layer.rs to wrap column builders in Arc<Mutex<ColBuilder>> and use Arc::try_unwrap for safe concurrent access, eliminating race conditions and preventing double-closes during pass2.

memory/MEMORY.md

@@ -2,3 +2,4 @@
 
 - [Project domain](project_domain.md) — obikmer est pour la génomique (génomes individuels), pas la métagénomique
 - [No architectural decisions without authorization](feedback_architectural_decisions.md) — toute décision architecturale (mémoire, algo, structure) requiert l'accord explicite de l'utilisateur avant toute action
+- [Phases intra-partition parallèles](feedback_phases_parallelism.md) — graph build, compute_degrees, unitig traversal, MPHF utilisent Rayon — ne jamais les appeler "séquentielles"

memory/feedback_phases_parallelism.md

@@ -0,0 +1,12 @@
+---
+name: feedback-phases-parallelism
+description: Les phases intra-partition (graph build, compute_degrees, unitig traversal, MPHF) utilisent toutes Rayon — elles ne sont PAS séquentielles
+metadata:
+  type: feedback
+---
+
+Ne jamais qualifier les phases intra-partition de "séquentielles". Chaque phase (graph build, compute_degrees, unitig traversal, MPHF build) utilise Rayon en interne et s'exécute en parallèle sur plusieurs cœurs.
+
+**Why:** L'utilisateur a corrigé ce point plusieurs fois. Le décrire comme "séquentiel" est une erreur factuelle qui fausse l'analyse de performance.
+
+**How to apply:** Quand on analyse l'efficacité CPU ou les 25% manquants, chercher la cause dans le déséquilibre de charge entre partitions, la contention Rayon entre workers, ou la latence inter-partitions — pas dans une prétendue sérialisation des phases.

src/obikpartitionner/src/merge_layer.rs

@@ -453,8 +453,18 @@ impl KmerPartition {
             WriteBatch(Vec<(Option<usize>, usize, usize, u32)>),
         }
 
-        let builders       = Arc::new(Mutex::new((exist_builders, new_src_builders)));
+        let exist_locked: Vec<Vec<Arc<Mutex<ColBuilder>>>> = exist_builders
-        let builders_sink  = Arc::clone(&builders);
+            .into_iter()
+            .map(|layer| layer.into_iter().map(|b| Arc::new(Mutex::new(b))).collect())
+            .collect();
+        let new_locked: Vec<Arc<Mutex<ColBuilder>>> = new_src_builders
+            .into_iter()
+            .map(|b| Arc::new(Mutex::new(b)))
+            .collect();
+        let exist_sink: Vec<Vec<Arc<Mutex<ColBuilder>>>> = exist_locked.iter()
+            .map(|layer| layer.iter().map(Arc::clone).collect())
+            .collect();
+        let new_sink: Vec<Arc<Mutex<ColBuilder>>> = new_locked.iter().map(Arc::clone).collect();
         let dst_map_t2     = Arc::clone(&dst_map);
         let new_mphf_t2    = new_mphf.clone();
         let pass2_err: Arc<Mutex<Option<String>>> = Arc::new(Mutex::new(None));
@@ -533,11 +543,10 @@ impl KmerPartition {
             ],
             make_sink!(Pass2Data, {
                 move |ops: Vec<(Option<usize>, usize, usize, u32)>| {
-                    let mut guard = builders_sink.lock().unwrap();
                     for (layer_opt, col, slot, val) in ops {
                         match layer_opt {
-                            Some(l) => guard.0[l][col].set_val(slot, val),
+                            Some(l) => exist_sink[l][col].lock().unwrap().set_val(slot, val),
-                            None    => guard.1[col].set_val(slot, val),
+                            None    => new_sink[col].lock().unwrap().set_val(slot, val),
                         }
                     }
                 }
@@ -555,17 +564,16 @@ impl KmerPartition {
             return Err(SKError::InvalidData { context: "merge pass2", detail: msg });
         }
 
-        let (exist_builders, new_src_builders) = Arc::try_unwrap(builders)
-            .unwrap_or_else(|_| panic!("pass2: builders not uniquely owned after pipeline"))
-            .into_inner()
-            .unwrap_or_else(|e| e.into_inner());
-
         let t_close = std::time::Instant::now();
         // ── Close builders and update metadata ────────────────────────────────
-        for (l, builders) in exist_builders.into_iter().enumerate() {
+        for (l, builders) in exist_locked.into_iter().enumerate() {
             let layer_dir = dst_index_dir.join(format!("layer_{l}"));
             for b in builders {
-                b.close()?;
+                Arc::try_unwrap(b)
+                    .unwrap_or_else(|_| panic!("pass2: exist_builder[{l}] not uniquely owned"))
+                    .into_inner()
+                    .unwrap_or_else(|e| e.into_inner())
+                    .close()?;
             }
             let n = dst_map.layer(l).n();
             let data_dir = match mode {
@@ -575,8 +583,12 @@ impl KmerPartition {
             write_matrix_meta(&data_dir, n, n_dst_genomes + n_src_total).map_err(SKError::Io)?;
         }
 
-        for b in new_src_builders {
+        for b in new_locked {
-            b.close()?;
+            Arc::try_unwrap(b)
+                .unwrap_or_else(|_| panic!("pass2: new_builder not uniquely owned"))
+                .into_inner()
+                .unwrap_or_else(|e| e.into_inner())
+                .close()?;
         }
         if any_new {
             let data_dir = match mode {