Phase 3d-A: SlabMeta Box boundary - Encapsulate SuperSlab metadata access

ChatGPT-guided Box theory refactoring (Phase A: Boundary only).

Changes:
- Created ss_slab_meta_box.h with 15 inline accessor functions
  - HOT fields (8): freelist, used, capacity (fast path)
  - COLD fields (6): class_idx, carved, owner_tid_low (init/debug)
  - Legacy (1): ss_slab_meta_ptr() for atomic ops
- Migrated 14 direct slabs[] access sites across 6 files
  - hakmem_shared_pool.c (4 sites)
  - tiny_free_fast_v2.inc.h (1 site)
  - hakmem_tiny.c (3 sites)
  - external_guard_box.h (1 site)
  - hakmem_tiny_lifecycle.inc (1 site)
  - ss_allocation_box.c (4 sites)

Architecture:
- Zero overhead (static inline wrappers)
- Single point of change for future layout optimizations
- Enables Hot/Cold split (Phase C) without touching call sites
- A/B testing support via compile-time flags

Verification:
- Build: ✅ Success (no errors)
- Stability: ✅ All sizes pass (128B-1KB, 22-24M ops/s)
- Behavior: Unchanged (thin wrapper, no logic changes)

Next: Phase B (TLS Cache Merge, +12-18% expected)

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

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

core/box/external_guard_box.h

@@ -25,6 +25,7 @@
 #include <stdlib.h>
 #include <sys/mman.h>
 #include "front_gate_v2.h"  // Phase 15: For fg_classification_t types
+#include "ss_slab_meta_box.h"  // Phase 3d-A: SlabMeta Box boundary
 
 // ENV control: mincore enable/disable
 static inline int external_guard_mincore_enabled(void) {
@@ -108,7 +109,7 @@ static inline int external_guard_try_free(void* ptr) {
         fprintf(stderr, "[ExternalGuard] hak_super_lookup(ptr) = %p\n", (void*)ss);
         if (ss) {
             fprintf(stderr, "[ExternalGuard] HAKMEM SS FOUND! ptr=%p ss=%p magic=0x%x class=%d\n",
-                    ptr, (void*)ss, ss->magic, ss->slabs ? ss->slabs[0].class_idx : -1);
+                    ptr, (void*)ss, ss->magic, ss->slabs ? ss_slab_meta_class_idx_get(ss, 0) : -1);
         }
 
         // Debug: Check FrontGate classification (types defined in front_gate_v2.h)