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/front/tiny_unified_cache.d

@@ -43,7 +43,7 @@ core/front/tiny_unified_cache.o: core/front/tiny_unified_cache.c \
  core/front/../box/warm_pool_prefill_box.h \
  core/front/../box/../tiny_tls.h \
  core/front/../box/../box/warm_pool_stats_box.h \
- core/front/../hakmem_env_cache.h
+ core/front/../hakmem_env_cache.h core/front/../box/tiny_page_box.h
 core/front/tiny_unified_cache.h:
 core/front/../hakmem_build_flags.h:
 core/front/../hakmem_tiny_config.h:
@@ -108,3 +108,4 @@ core/front/../box/warm_pool_prefill_box.h:
 core/front/../box/../tiny_tls.h:
 core/front/../box/../box/warm_pool_stats_box.h:
 core/front/../hakmem_env_cache.h:
+core/front/../box/tiny_page_box.h: