Phase 12: Shared SuperSlab Pool implementation (WIP - runtime crash)

## Summary
Implemented Phase 12 Shared SuperSlab Pool (mimalloc-style) to address
SuperSlab allocation churn (877 SuperSlabs → 100-200 target).

## Implementation (ChatGPT + Claude)
1. **Metadata changes** (superslab_types.h):
   - Added class_idx to TinySlabMeta (per-slab dynamic class)
   - Removed size_class from SuperSlab (no longer per-SuperSlab)
   - Changed owner_tid (16-bit) → owner_tid_low (8-bit)

2. **Shared Pool** (hakmem_shared_pool.{h,c}):
   - Global pool shared by all size classes
   - shared_pool_acquire_slab() - Get free slab for class_idx
   - shared_pool_release_slab() - Return slab when empty
   - Per-class hints for fast path optimization

3. **Integration** (23 files modified):
   - Updated all ss->size_class → meta->class_idx
   - Updated all meta->owner_tid → meta->owner_tid_low
   - superslab_refill() now uses shared pool
   - Free path releases empty slabs back to pool

4. **Build system** (Makefile):
   - Added hakmem_shared_pool.o to OBJS_BASE and TINY_BENCH_OBJS_BASE

## Status: ⚠️ Build OK, Runtime CRASH

**Build**: ✅ SUCCESS
- All 23 files compile without errors
- Only warnings: superslab_allocate type mismatch (legacy code)

**Runtime**: ❌ SEGFAULT
- Crash location: sll_refill_small_from_ss()
- Exit code: 139 (SIGSEGV)
- Test case: ./bench_random_mixed_hakmem 1000 256 42

## Known Issues
1. **SEGFAULT in refill path** - Likely shared_pool_acquire_slab() issue
2. **Legacy superslab_allocate()** still exists (type mismatch warning)
3. **Remaining TODOs** from design doc:
   - SuperSlab physical layout integration
   - slab_handle.h cleanup
   - Remove old per-class head implementation

## Next Steps
1. Debug SEGFAULT (gdb backtrace shows sll_refill_small_from_ss)
2. Fix shared_pool_acquire_slab() or superslab_init_slab()
3. Basic functionality test (1K → 100K iterations)
4. Measure SuperSlab count reduction (877 → 100-200)
5. Performance benchmark (+650-860% expected)

## Files Changed (25 files)
core/box/free_local_box.c
core/box/free_remote_box.c
core/box/front_gate_classifier.c
core/hakmem_super_registry.c
core/hakmem_tiny.c
core/hakmem_tiny_bg_spill.c
core/hakmem_tiny_free.inc
core/hakmem_tiny_lifecycle.inc
core/hakmem_tiny_magazine.c
core/hakmem_tiny_query.c
core/hakmem_tiny_refill.inc.h
core/hakmem_tiny_superslab.c
core/hakmem_tiny_superslab.h
core/hakmem_tiny_tls_ops.h
core/slab_handle.h
core/superslab/superslab_inline.h
core/superslab/superslab_types.h
core/tiny_debug.h
core/tiny_free_fast.inc.h
core/tiny_free_magazine.inc.h
core/tiny_remote.c
core/tiny_superslab_alloc.inc.h
core/tiny_superslab_free.inc.h
Makefile

## New Files (3 files)
PHASE12_SHARED_SUPERSLAB_POOL_DESIGN.md
core/hakmem_shared_pool.c
core/hakmem_shared_pool.h

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

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

core/hakmem_tiny_query.c

@@ -33,12 +33,9 @@ int hak_tiny_is_managed_superslab(void* ptr) {
     // Safety: Only check if g_use_superslab is enabled
     if (g_use_superslab) {
         SuperSlab* ss = hak_super_lookup(ptr);
-        // Phase 8.2 optimization: Use alignment check instead of mincore()
-        // SuperSlabs are always SUPERSLAB_SIZE-aligned (2MB)
-        if (ss && ((uintptr_t)ss & (SUPERSLAB_SIZE - 1)) == 0) {
-            if (ss->magic == SUPERSLAB_MAGIC) {
-                return 1;  // Valid SuperSlab pointer
-            }
+        if (ss && ss->magic == SUPERSLAB_MAGIC) {
+            // Phase 12: Use registry hit as "managed by SuperSlab"
+            return 1;
         }
     }
 
@@ -53,16 +50,20 @@ size_t hak_tiny_usable_size(void* ptr) {
     if (g_use_superslab) {
         SuperSlab* ss = hak_super_lookup(ptr);
         if (ss && ss->magic == SUPERSLAB_MAGIC) {
-            int k = (int)ss->size_class;
-            if (k >= 0 && k < TINY_NUM_CLASSES) {
-                // Phase E1-CORRECT: g_tiny_class_sizes = total size (stride)
-                // Usable = stride - 1 (for 1-byte header)
+            // Phase 12: derive class from per-slab meta (no ss->size_class)
+            void* base = (void*)((uint8_t*)ptr - 1);
+            int slab_idx = slab_index_for(ss, base);
+            if (slab_idx >= 0 && slab_idx < ss_slabs_capacity(ss)) {
+                TinySlabMeta* meta = &ss->slabs[slab_idx];
+                int k = (meta->class_idx < TINY_NUM_CLASSES) ? (int)meta->class_idx : -1;
+                if (k >= 0) {
 #if HAKMEM_TINY_HEADER_CLASSIDX
-                size_t stride = g_tiny_class_sizes[k];
-                return (stride > 0) ? (stride - 1) : 0;
+                    size_t stride = g_tiny_class_sizes[k];
+                    return (stride > 0) ? (stride - 1) : 0;
 #else
-                return g_tiny_class_sizes[k];
+                    return g_tiny_class_sizes[k];
 #endif
+                }
             }
         }
     }