Phase v4-mid-0: Small-object v4 型・IF 足場（箱化モジュール化）

- SmallHeapCtx/SmallPageMeta/SmallClassHeap typedef alias 追加
- SmallSegment struct (base/num_pages/owner_tid/magic) を smallsegment_v4_box.h に定義
- SmallColdIface_v4 direct function prototypes (refill/retire/remote_push/drain)
- smallobject_hotbox_v4.c の internal/public API 分離（small_segment_v4_internal）
- direct function stubs 実装（SmallColdIfaceV4 delegate 形式）
- ENV OFF デフォルト（ENABLED=0/CLASSES=0）で既存挙動 100% 不変
- ビルド成功・sanity 確認（mixed/C6-heavy、segv/assert なし）
- CURRENT_TASK.md に Phase v4-mid-0 記録

🤖 Generated with Claude Code

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

core/smallobject_hotbox_v4.c

@@ -19,13 +19,14 @@
 // TLS context
 static __thread small_heap_ctx_v4 g_ctx_v4;
 
-typedef struct small_segment_v4 {
+// Internal segment structure (internal use only, not exposed via public box API)
+typedef struct small_segment_v4_internal {
     int class_idx;
     size_t segment_size;
     tiny_heap_ctx_t* tiny_ctx;
-} small_segment_v4;
+} small_segment_v4_internal;
 
-static __thread small_segment_v4 g_segments_v4[SMALLOBJECT_NUM_CLASSES];
+static __thread small_segment_v4_internal g_segments_v4[SMALLOBJECT_NUM_CLASSES];
 
 small_heap_ctx_v4* small_heap_ctx_v4_get(void) {
     return &g_ctx_v4;
@@ -54,7 +55,7 @@ static size_t smallsegment_v4_default_size(void) {
 
 small_segment_v4* smallsegment_v4_acquire(int class_idx) {
     if (!v4_class_supported(class_idx)) return NULL;
-    small_segment_v4* seg = &g_segments_v4[class_idx];
+    small_segment_v4_internal* seg = &g_segments_v4[class_idx];
     seg->class_idx = class_idx;
     if (!seg->segment_size) {
         seg->segment_size = smallsegment_v4_default_size();
@@ -62,27 +63,29 @@ small_segment_v4* smallsegment_v4_acquire(int class_idx) {
     if (!seg->tiny_ctx) {
         seg->tiny_ctx = tiny_heap_ctx_for_thread();
     }
-    return seg;
+    return (small_segment_v4*)seg;
 }
 
 void* smallsegment_v4_alloc_page(small_segment_v4* seg, int class_idx) {
     if (!seg || !v4_class_supported(class_idx)) return NULL;
-    if (!seg->tiny_ctx) {
-        seg->tiny_ctx = tiny_heap_ctx_for_thread();
+    // Internal use only: cast to internal type to access tiny_ctx
+    small_segment_v4_internal* int_seg = (small_segment_v4_internal*)seg;
+    if (!int_seg->tiny_ctx) {
+        int_seg->tiny_ctx = tiny_heap_ctx_for_thread();
     }
-    tiny_heap_ctx_t* tctx = seg->tiny_ctx ? seg->tiny_ctx : tiny_heap_ctx_for_thread();
+    tiny_heap_ctx_t* tctx = int_seg->tiny_ctx ? int_seg->tiny_ctx : tiny_heap_ctx_for_thread();
     if (!tctx) return NULL;
 
     tiny_heap_page_t* lease = tiny_heap_prepare_page(tctx, class_idx);
     if (!lease) return NULL;
-    seg->tiny_ctx = tctx;
+    int_seg->tiny_ctx = tctx;
     return v4_page_from_lease(lease, class_idx, seg);
 }
 
 void smallsegment_v4_release_if_empty(small_segment_v4* seg, void* page_ptr, int class_idx) {
     small_page_v4* page = (small_page_v4*)page_ptr;
     if (!page || !v4_class_supported(class_idx)) return;
-    tiny_heap_ctx_t* tctx = (seg && seg->tiny_ctx) ? seg->tiny_ctx : tiny_heap_ctx_for_thread();
+    tiny_heap_ctx_t* tctx = tiny_heap_ctx_for_thread();
     tiny_heap_page_t* lease = (tiny_heap_page_t*)page->slab_ref;
     if (tctx && lease) {
         tiny_heap_page_becomes_empty(tctx, class_idx, lease);
@@ -256,6 +259,40 @@ const SmallColdIfaceV4* small_cold_iface_v4_get(void) {
     return &g_cold_iface_v4;
 }
 
+// Direct function stubs (phase v4-mid-0: delegates to iface)
+small_page_v4* small_cold_v4_refill_page(small_heap_ctx_v4* ctx, uint32_t class_idx) {
+    const SmallColdIfaceV4* iface = small_cold_iface_v4_get();
+    if (!iface || !iface->refill_page) return NULL;
+    return iface->refill_page(ctx, class_idx);
+}
+
+void small_cold_v4_retire_page(small_heap_ctx_v4* ctx, small_page_v4* page) {
+    const SmallColdIfaceV4* iface = small_cold_iface_v4_get();
+    if (!iface || !iface->retire_page || !page) return;
+    iface->retire_page(ctx, (uint32_t)page->class_idx, page);
+}
+
+bool small_cold_v4_remote_push(small_page_v4* page, void* ptr, uint32_t tid) {
+    const SmallColdIfaceV4* iface = small_cold_iface_v4_get();
+    if (!iface || !iface->remote_push) return false;
+    return iface->remote_push(page, ptr, tid);
+}
+
+void small_cold_v4_remote_drain(small_heap_ctx_v4* ctx) {
+    const SmallColdIfaceV4* iface = small_cold_iface_v4_get();
+    if (!iface || !iface->remote_drain) return;
+    iface->remote_drain(ctx);
+}
+
+// Stub accessor for smallsegment_v4_page_meta_of
+small_page_v4* smallsegment_v4_page_meta_of(small_segment_v4* seg, void* ptr) {
+    // Phase v4-mid-0: stub only, will be implemented in later phase
+    // For now, return NULL (unimplemented)
+    (void)seg;
+    (void)ptr;
+    return NULL;
+}
+
 // -----------------------------------------------------------------------------
 // alloc/free
 // -----------------------------------------------------------------------------