Complete Phase 1.2: Centralize layout definitions in tiny_layout_box.h

Changes:
- Updated ptr_conversion_box.h: Use TINY_HEADER_SIZE instead of hardcoded -1
- Updated tiny_front_hot_box.h: Use tiny_user_offset() for BASE->USER conversion
- Updated tiny_front_cold_box.h: Use tiny_user_offset() for BASE->USER conversion
- Added tiny_layout_box.h includes to both front box headers

Box theory: Layout parameters now isolated in dedicated Box component.
All offset arithmetic centralized - no scattered +1/-1 arithmetic.

Verified: Build succeeds (make clean && make shared -j8)

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

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

core/box/tiny_front_cold_box.h

@@ -25,6 +25,7 @@
 #include "../hakmem_tiny_config.h"
 #include "../tiny_region_id.h"
 #include "../front/tiny_unified_cache.h"  // For TinyUnifiedCache, unified_cache_refill
+#include "tiny_layout_box.h"  // For tiny_user_offset()
 
 // ============================================================================
 // Box 3: Tiny Cold Refill + Alloc
@@ -76,7 +77,9 @@ static inline void* tiny_cold_refill_and_alloc(int class_idx) {
     // NOTE: Header already written by unified_cache_refill()
     // (Removed redundant tiny_region_id_write_header() - P2 fix)
     #if HAKMEM_TINY_HEADER_CLASSIDX
-    return (void*)((char*)base + 1);  // USER pointer
+    // Use centralized layout API for offset calculation
+    size_t user_offset = tiny_user_offset(class_idx);
+    return (void*)((char*)base + user_offset);  // USER pointer
     #else
     return base;
     #endif