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_legacy_backend_box.h

@@ -0,0 +1,43 @@
+// Box: Legacy Backend (Phase 12)
+// Purpose: Per-class SuperSlabHead backend (legacy implementation)
+//
+// Responsibilities:
+//   - Maintain per-class SuperSlabHead (g_superslab_heads)
+//   - Initialize SuperSlabHead for a class
+//   - Expand SuperSlabHead by allocating new chunks
+//   - Find chunk for a pointer (chunk walk)
+//   - Legacy hint box (per-thread, per-class bump allocation)
+//
+// Dependencies:
+//   - ss_allocation_box (superslab_allocate, superslab_free, superslab_init_slab)
+//   - hakmem_tiny_config (g_tiny_class_sizes)
+//
+// API:
+//   - init_superslab_head() - initialize SuperSlabHead for a class
+//   - expand_superslab_head() - expand SuperSlabHead by allocating new chunk
+//   - find_chunk_for_ptr() - find chunk for a pointer
+//   - hak_tiny_alloc_superslab_backend_legacy() - per-class backend
+//   - hak_tiny_alloc_superslab_backend_hint() - hint optimization
+//   - hak_tiny_ss_hint_record() - hint recording
+
+#ifndef SS_LEGACY_BACKEND_BOX_H
+#define SS_LEGACY_BACKEND_BOX_H
+
+#include "hakmem_tiny_superslab.h"
+
+// Global per-class SuperSlabHeads
+extern SuperSlabHead* g_superslab_heads[TINY_NUM_CLASSES_SS];
+
+// SuperSlabHead management
+SuperSlabHead* init_superslab_head(int class_idx);
+int expand_superslab_head(SuperSlabHead* head);
+SuperSlab* find_chunk_for_ptr(void* ptr, int class_idx);
+
+// Legacy backend API
+void* hak_tiny_alloc_superslab_backend_legacy(int class_idx);
+
+// Hint box (optional optimization)
+void* hak_tiny_alloc_superslab_backend_hint(int class_idx);
+void hak_tiny_ss_hint_record(int class_idx, SuperSlab* ss, int slab_idx);
+
+#endif // SS_LEGACY_BACKEND_BOX_H