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

core/box/c7_hotpath_env_box.h

@@ -0,0 +1,15 @@
+// c7_hotpath_env_box.h - ENV gate for C7 hotpath
+// Purpose: isolate the ENV handling so hotpath code can assume gate済み。
+#pragma once
+
+#include <stdlib.h>
+
+// ENV gate: HAKMEM_TINY_C7_HOT=1 で有効化（デフォルト OFF）
+static inline int tiny_c7_hot_enabled(void) {
+    static int g_enable = -1;
+    if (__builtin_expect(g_enable == -1, 0)) {
+        const char* e = getenv("HAKMEM_TINY_C7_HOT");
+        g_enable = (e && *e && *e != '0') ? 1 : 0;
+    }
+    return g_enable;
+}