Phase v4-3.1: reuse C7 v4 pages and record prep calls

core/box/smallobject_cold_iface_v4.h

@@ -0,0 +1,21 @@
+// smallobject_cold_iface_v4.h - SmallObject HotHeap v4 Cold Interface (境界 API のみ)
+//
+// 役割:
+//   - HotBox_v4 と Superslab/Warm/Remote を繋ぐ関数ポインタの箱を定義する。
+//   - 実装は後続フェーズで追加し、いまは型と宣言だけを置く。
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "smallobject_hotbox_v4_box.h"
+
+typedef struct SmallColdIfaceV4 {
+    small_page_v4* (*refill_page)(small_heap_ctx_v4*, uint32_t class_idx);
+    void (*retire_page)(small_heap_ctx_v4*, uint32_t class_idx, small_page_v4* page);
+    bool (*remote_push)(small_page_v4* page, void* ptr, uint32_t self_tid);
+    void (*remote_drain)(small_heap_ctx_v4*);
+} SmallColdIfaceV4;
+
+// Cold iface accessor（実装は後続フェーズ）
+const SmallColdIfaceV4* small_cold_iface_v4_get(void);