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/tiny_free_fast.inc.h

@@ -38,8 +38,8 @@ extern uint32_t tiny_self_u32(void);
 extern pthread_t tiny_self_pt(void);
 
 // External TLS variables (from Box 5)
-extern __thread void* g_tls_sll_head[TINY_NUM_CLASSES];
-extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
+// Phase 3d-B: TLS Cache Merge - Unified TLS SLL structure
+extern __thread TinyTLSSLL g_tls_sll[TINY_NUM_CLASSES];
 // Hot-class toggle: class5 (256B) dedicated TLS fast path
 extern int g_tiny_hotpath_class5;
 extern __thread TinyTLSList g_tls_lists[TINY_NUM_CLASSES];