Phase 3d-B: TLS Cache Merge - Unified g_tls_sll[] structure (+12-18% expected)

Merge separate g_tls_sll_head[] and g_tls_sll_count[] arrays into unified
TinyTLSSLL struct to improve L1D cache locality. Expected performance gain:
+12-18% from reducing cache line splits (2 loads → 1 load per operation).

Changes:
- core/hakmem_tiny.h: Add TinyTLSSLL type (16B aligned, head+count+pad)
- core/hakmem_tiny.c: Replace separate arrays with g_tls_sll[8]
- core/box/tls_sll_box.h: Update Box API (13 sites) for unified access
- Updated 32+ files: All g_tls_sll_head[i] → g_tls_sll[i].head
- Updated 32+ files: All g_tls_sll_count[i] → g_tls_sll[i].count
- core/hakmem_tiny_integrity.h: Unified canary guards
- core/box/integrity_box.c: Simplified canary validation
- Makefile: Added core/box/tiny_sizeclass_hist_box.o to link

Build: ✅ PASS (10K ops sanity test)
Warnings: Only pre-existing LTO type mismatches (unrelated)

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

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

core/box/ss_unified_backend_box.h

@@ -0,0 +1,29 @@
+// Box: Unified Backend (Phase 12)
+// Purpose: Unified entry point for SuperSlab allocation (shared pool + legacy fallback)
+//
+// Responsibilities:
+//   - Single front-door for tiny-side SuperSlab allocations
+//   - Shared pool backend integration
+//   - Optional legacy fallback for compatibility
+//   - ENV-based policy control (HAKMEM_TINY_SS_SHARED, etc.)
+//
+// Dependencies:
+//   - ss_legacy_backend_box (legacy backend)
+//   - hakmem_shared_pool (shared pool backend)
+//
+// API:
+//   - hak_tiny_alloc_superslab_box() - unified entry point
+//   - hak_tiny_alloc_superslab_backend_shared() - shared pool backend
+
+#ifndef SS_UNIFIED_BACKEND_BOX_H
+#define SS_UNIFIED_BACKEND_BOX_H
+
+#include <stddef.h>
+
+// Unified entry point (Box API)
+void* hak_tiny_alloc_superslab_box(int class_idx);
+
+// Shared pool backend
+void* hak_tiny_alloc_superslab_backend_shared(int class_idx);
+
+#endif // SS_UNIFIED_BACKEND_BOX_H