Mid-Large Phase 12 Complete + P0-5 Lock-Free Stage 2

**Phase 12 第1ラウンド完了** ✅
- 0.24M → 2.39M ops/s (8T, **+896%**)
- SEGFAULT → Zero crashes (**100% → 0%**)
- futex: 209 → 10 calls (**-95%**)

**P0-5: Lock-Free Stage 2 (Slot Claiming)**
- Atomic SlotState: `_Atomic SlotState state`
- sp_slot_claim_lockfree(): CAS-based UNUSED→ACTIVE transition
- acquire_slab() Stage 2: Lock-free claiming (mutex only for metadata)
- Result: 2.34M → 2.39M ops/s (+2.5% @ 8T)

**Implementation**:
- core/hakmem_shared_pool.h: Atomic SlotState definition
- core/hakmem_shared_pool.c:
  - sp_slot_claim_lockfree() (+40 lines)
  - Atomic helpers: sp_slot_find_unused/mark_active/mark_empty
  - Stage 2 lock-free integration
- Verified via debug logs: STAGE2_LOCKFREE claiming works

**Reports**:
- MID_LARGE_P0_PHASE_REPORT.md: P0-0 to P0-4 comprehensive summary
- MID_LARGE_FINAL_AB_REPORT.md: Complete Phase 12 A/B comparison (17KB)
  - Performance evolution table
  - Lock contention analysis  - Lessons learned
  - File inventory

**Tiny Baseline Measurement** 📊
- System malloc: 82.9M ops/s (256B)
- HAKMEM:        8.88M ops/s (256B)
- **Gap: 9.3x slower** (target for next phase)

**Next**: Tiny allocator optimization (drain interval, front cache, perf profile)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/hakmem_shared_pool.h

@@ -40,10 +40,11 @@ typedef enum {
 } SlotState;
 
 // Per-slot metadata
+// P0-5: state is atomic for lock-free claiming
 typedef struct {
-    SlotState state;
-    uint8_t   class_idx;  // Valid when state != SLOT_UNUSED (0-7)
-    uint8_t   slab_idx;   // SuperSlab-internal index (0-31)
+    _Atomic SlotState state;  // Atomic for lock-free CAS (UNUSED→ACTIVE)
+    uint8_t   class_idx;      // Valid when state != SLOT_UNUSED (0-7)
+    uint8_t   slab_idx;       // SuperSlab-internal index (0-31)
 } SharedSlot;
 
 // Per-SuperSlab metadata for slot management
@@ -56,6 +57,31 @@ typedef struct SharedSSMeta {
     struct SharedSSMeta* next;               // For free list linking
 } SharedSSMeta;
 
+// ============================================================================
+// P0-4: Lock-Free Free Slot List (LIFO Stack)
+// ============================================================================
+
+// Free slot node for lock-free linked list
+typedef struct FreeSlotNode {
+    SharedSSMeta* meta;       // Which SuperSlab metadata
+    uint8_t       slot_idx;   // Which slot within that SuperSlab
+    struct FreeSlotNode* next;  // Next node in LIFO stack
+} FreeSlotNode;
+
+// Lock-free per-class free slot list (LIFO stack with atomic head)
+typedef struct {
+    _Atomic(FreeSlotNode*) head;  // Atomic stack head pointer
+} LockFreeFreeList;
+
+// Node pool for lock-free allocation (avoid malloc/free)
+#define MAX_FREE_NODES_PER_CLASS 512  // Pre-allocated nodes per class
+extern FreeSlotNode g_free_node_pool[TINY_NUM_CLASSES_SS][MAX_FREE_NODES_PER_CLASS];
+extern _Atomic uint32_t g_node_alloc_index[TINY_NUM_CLASSES_SS];
+
+// ============================================================================
+// Legacy Free Slot List (for comparison, will be removed after P0-4)
+// ============================================================================
+
 // Free slot entry for per-class reuse lists
 typedef struct {
     SharedSSMeta* meta;   // Which SuperSlab metadata
@@ -87,7 +113,10 @@ typedef struct SharedSuperSlabPool {
     uint32_t    lru_count;
 
     // ========== Phase 12: SP-SLOT Management ==========
-    // Per-class free slot lists for efficient reuse
+    // P0-4: Lock-free per-class free slot lists (atomic LIFO stacks)
+    LockFreeFreeList free_slots_lockfree[TINY_NUM_CLASSES_SS];
+
+    // Legacy: Per-class free slot lists (mutex-protected, for comparison)
     FreeSlotList free_slots[TINY_NUM_CLASSES_SS];
 
     // SharedSSMeta array for all SuperSlabs in pool