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/front_gate_box.c

@@ -5,8 +5,7 @@
 #include "ptr_conversion_box.h"  // Box 3: Pointer conversions
 
 // TLS SLL state (extern from hakmem_tiny.c)
-extern __thread void* g_tls_sll_head[TINY_NUM_CLASSES];
-extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
+extern __thread TinyTLSSLL g_tls_sll[TINY_NUM_CLASSES];
 extern int g_tls_sll_enable;  // set at init via HAKMEM_TINY_TLS_SLL
 
 // Front breakdown counters (extern from hakmem_tiny.c)
@@ -53,7 +52,7 @@ void front_gate_after_refill(int class_idx, int refilled_count) {
     int to_move = refilled_count / 2;
     if (to_move <= 0) return;
 
-    while (to_move-- > 0 && g_tls_sll_count[class_idx] > 0) {
+    while (to_move-- > 0 && g_tls_sll[class_idx].count > 0) {
         // SLL pop
         void* ptr = NULL;
         if (!tls_sll_pop(class_idx, &ptr)) break;