Phase 9-2: Remove Legacy Backend & Unify to Shared Pool (50M ops/s)

- Removed Legacy Backend fallback; Shared Pool is now the sole backend.
- Removed Soft Cap limit in Shared Pool to allow full memory management.
- Implemented EMPTY slab recycling with batched meta->used decrement in remote drain.
- Updated tiny_free_local_box to return is_empty status for safe recycling.
- Fixed race condition in release path by removing from legacy list early.
- Achieved 50.3M ops/s in WS8192 benchmark (+200% vs baseline).

core/hakmem_shared_pool_acquire.c

@@ -418,19 +418,8 @@ stage2_fallback:
     }
 
     // Before creating a new SuperSlab, consult learning-layer soft cap.
-    // If current active slots for this class already exceed the policy cap,
-    // fail early so caller can fall back to legacy backend.
-    uint32_t limit = sp_class_active_limit(class_idx);
-    if (limit > 0) {
-        uint32_t cur = g_shared_pool.class_active_slots[class_idx];
-        if (cur >= limit) {
-            if (g_lock_stats_enabled == 1) {
-                atomic_fetch_add(&g_lock_release_count, 1);
-            }
-            pthread_mutex_unlock(&g_shared_pool.alloc_lock);
-            return -1;  // Soft cap reached for this class
-        }
-    }
+    // Phase 9-2: Soft Cap removed to allow Shared Pool to fully replace Legacy Backend.
+    // We now rely on LRU eviction and EMPTY recycling to manage memory pressure.
 
     // Create metadata for this new SuperSlab
     SharedSSMeta* new_meta = sp_meta_find_or_create(new_ss);