Phase REFACTOR-1: Magic Number → Named Constants (TINY_CLASS_C6/C7)

Replace hardcoded class_idx checks (== 6, == 7) with named macros:
- tiny_class_is_c6(idx) for C6 checks
- tiny_class_is_c7(idx) for C7 checks
- tiny_class_is_ultra(idx) for combined checks

Benefits:
- Self-documenting code (semantic intent is clear)
- Single source of truth for class constants
- Easier to extend to other ULTRA tiers (C5, C8) in future

Changes:
- NEW: core/box/tiny_ultra_classes_box.h (named constants + helpers)
- Modified: core/front/malloc_tiny_fast.h (4 replacements: L181, L193, L326, L337)

No performance impact (zero-cost macros, same compiled code).

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

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

core/box/tiny_ultra_classes_box.h

@@ -0,0 +1,13 @@
+#ifndef HAKMEM_TINY_ULTRA_CLASSES_BOX_H
+#define HAKMEM_TINY_ULTRA_CLASSES_BOX_H
+
+// Purpose: Named constants for ULTRA tier classes (C6, C7)
+#define TINY_CLASS_C6 6
+#define TINY_CLASS_C7 7
+
+// Helper macros for class checking
+#define tiny_class_is_c6(idx)    ((idx) == TINY_CLASS_C6)
+#define tiny_class_is_c7(idx)    ((idx) == TINY_CLASS_C7)
+#define tiny_class_is_ultra(idx) (tiny_class_is_c6(idx) || tiny_class_is_c7(idx))
+
+#endif // HAKMEM_TINY_ULTRA_CLASSES_BOX_H