Feat(phase9): Safe removal from legacy list on shared pool free (Task 2)

Added remove_superslab_from_legacy_head to safely unlink SuperSlabs from
legacy g_superslab_heads when freed by shared_pool_release_slab.
This prevents dangling pointers in the legacy backend if fallback allocation was used.
Called after unlocking alloc_lock to avoid lock inversion.

core/hakmem_shared_pool.c

@@ -1489,6 +1489,10 @@ shared_pool_release_slab(SuperSlab* ss, int slab_idx)
 
         pthread_mutex_unlock(&g_shared_pool.alloc_lock);
 
+        // Remove from legacy backend list (if present) to prevent dangling pointers
+        extern void remove_superslab_from_legacy_head(SuperSlab* ss);
+        remove_superslab_from_legacy_head(ss);
+
         // Free SuperSlab:
         // 1. Try LRU cache (hak_ss_lru_push) - lazy deallocation
         // 2. Or munmap if LRU is full - eager deallocation