Add Page Box layer for C7 class optimization

- Implement tiny_page_box.c/h: per-thread page cache between UC and Shared Pool
- Integrate Page Box into Unified Cache refill path
- Remove legacy SuperSlab implementation (merged into smallmid)
- Add HAKMEM_TINY_PAGE_BOX_CLASSES env var for selective class enabling
- Update bench_random_mixed.c with Page Box statistics

Current status: Implementation safe, no regressions.
Page Box ON/OFF shows minimal difference - pool strategy needs tuning.

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

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

core/box/tiny_page_box.c

@@ -0,0 +1,6 @@
+#include "tiny_page_box.h"
+
+// TLS state definitions for Tiny Page Box
+__thread TinyPageBoxState g_tiny_page_box_state[TINY_NUM_CLASSES];
+__thread int g_tiny_page_box_init_done = 0;
+