Boxify superslab registry, add bench profile, and document C7 hotpath experiments

core/box/ss_budget_box.h

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+// ss_budget_box.h - Superslab Budget Box
+// Box Theory: centralize budget/limit checks for Superslab allocations.
+// Responsibilities:
+//   - Read budget ENV once (global + per-class override)
+//   - Provide cheap checks before allocating new Superslabs
+//   - Allow symmetric free hook for future accounting
+
+#ifndef HAKMEM_SS_BUDGET_BOX_H
+#define HAKMEM_SS_BUDGET_BOX_H
+
+#include <stdbool.h>
+
+// Return false when allocation should be denied due to budget exhaustion.
+bool ss_budget_on_alloc(int class_idx);
+
+// Hook for future bookkeeping; currently a no-op placeholder.
+void ss_budget_on_free(int class_idx);
+
+#endif // HAKMEM_SS_BUDGET_BOX_H