Phase 36-37: TinyHotHeap v2 HotBox redesign and C7 current_page policy fixes

- Redefine TinyHotHeap v2 as per-thread Hot Box with clear boundaries
- Add comprehensive OS statistics tracking for SS allocations
- Implement route-based free handling for TinyHeap v2
- Add C6/C7 debugging and statistics improvements
- Update documentation with implementation guidelines and analysis
- Add new box headers for stats, routing, and front-end management

core/box/ss_cache_box.c

@@ -159,6 +159,7 @@ void tiny_ss_cache_set_class_cap(int class_idx, size_t new_cap) {
             // Convert cache entry back to SuperSlab* and release it to OS.
             SuperSlab* ss = (SuperSlab*)entry;
             size_t ss_size = (size_t)1 << ss->lg_size;
+            ss_os_stats_record_free();
             munmap((void*)ss, ss_size);
 
             // Update global stats to keep accounting consistent.