Add v4 C7/C6 fast classify and small-segment v4 scaffolding

core/box/smallsegment_v4_env_box.h

@@ -0,0 +1,28 @@
+// smallsegment_v4_env_box.h - small-object segment v4 の ENV ゲート
+// Phase PF2: ENV を宣言するだけ。実装は次フェーズで追加。
+#pragma once
+
+#include <stdlib.h>
+
+static inline int smallsegment_v4_enabled(void) {
+    static int g = -1;
+    if (__builtin_expect(g == -1, 0)) {
+        const char* e = getenv("HAKMEM_SMALL_SEGMENT_V4_ENABLED");
+        if (e && *e && *e != '0') {
+            g = 1;
+        } else {
+            g = 0;
+        }
+    }
+    return g;
+}
+
+static inline const char* smallsegment_v4_size_env(void) {
+    static int g_init = 0;
+    static const char* g_val = NULL;
+    if (__builtin_expect(!g_init, 0)) {
+        g_val = getenv("HAKMEM_SMALL_SEGMENT_V4_SIZE");
+        g_init = 1;
+    }
+    return g_val;
+}