Cleanup: Unify type naming and Cold Iface architecture

Refactoring:
- Type naming: Rename small_page_v4 → SmallPageMeta, small_class_heap_v4 → SmallClassHeap, small_heap_ctx_v4 → SmallHeapCtx
- Keep backward compatibility aliases for existing code
- SmallSegment struct unified, clean forward declarations
- Cold Iface: Remove vtable (SmallColdIfaceV4 struct) in favor of direct function calls
- Simplify refill_page/retire_page to direct calls, not callbacks
- smallobject_hotbox_v4.c: Update to call small_cold_v4_* functions directly

Documentation:
- Add docs/analysis/ENV_CLEANUP_CANDIDATES.md
- Categorize ENVs: KEEP (production), RESEARCH (opt-in), DELETE (obsolete)
- v2 code: Keep as research infrastructure (complete, safe, gated)
- v4 code: Research scaffold for future mid-level allocator

Build:
- ビルド成功（警告のみ）
- Backward compatible, all existing code still works

🤖 Generated with Claude Code

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

core/box/smallobject_cold_iface_v4.h

@@ -1,8 +1,9 @@
-// smallobject_cold_iface_v4.h - SmallObject HotHeap v4 Cold Interface (境界 API のみ)
+// smallobject_cold_iface_v4.h - SmallObject HotHeap v4 Cold Interface (Direct Functions)
 //
 // 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。
+//   - HotBox_v4 と Superslab/Warm/Remote を繋ぐ Cold 側のインターフェース。
+//   - 関数ポインタ（vtable）ではなく direct function で統一。
+//   - Hot Box は以下の関数を直接呼び出す（callback ではなく）。
 #pragma once
 
 #include <stdbool.h>
@@ -10,17 +11,8 @@
 
 #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);
-
-// Direct function prototypes (Alternative API for direct calls, not yet implemented)
+// Cold path API: Direct functions (not function pointers)
+// Hot box から直接呼び出される Cold 側の実装。
 small_page_v4* small_cold_v4_refill_page(small_heap_ctx_v4* ctx, uint32_t class_idx);
 void small_cold_v4_retire_page(small_heap_ctx_v4* ctx, small_page_v4* page);
 bool small_cold_v4_remote_push(small_page_v4* page, void* ptr, uint32_t tid);

core/box/smallobject_hotbox_v4_box.h

@@ -15,10 +15,14 @@
 #define SMALLOBJECT_NUM_CLASSES TINY_NUM_CLASSES
 #endif
 
-struct small_segment_v4;
+struct SmallSegment;  // Forward declaration, defined in smallsegment_v4_box.h
 
-// Page metadata for v4 HotBox
-typedef struct small_page_v4 {
+// ============================================================================
+// SmallObject v4 HotBox Type Definitions
+// ============================================================================
+
+// SmallPageMeta: Page metadata for v4 HotBox
+typedef struct SmallPageMeta {
     void* freelist;
     uint16_t used;
     uint16_t capacity;
@@ -26,29 +30,28 @@ typedef struct small_page_v4 {
     uint8_t flags;
     uint32_t block_size;
     uint8_t* base;
-    void* slab_ref;  // Superslab / lease token (box境界で扱う)
-    struct small_segment_v4* segment;  // PF3: segment owner（NULLなら Tiny v1 経路）
-    struct small_page_v4* next;
-} small_page_v4;
+    void* slab_ref;  // Superslab / lease token (box boundary)
+    struct SmallSegment* segment;  // Segment owner (NULL if Tiny v1 path)
+    struct SmallPageMeta* next;
+} SmallPageMeta;
 
-// Per-class heap state (current / partial / full lists)
-typedef struct small_class_heap_v4 {
-    small_page_v4* current;
-    small_page_v4* partial_head;
-    small_page_v4* full_head;
+// SmallClassHeap: Per-class heap state (current / partial / full lists)
+typedef struct SmallClassHeap {
+    SmallPageMeta* current;
+    SmallPageMeta* partial_head;
+    SmallPageMeta* full_head;
     uint32_t partial_count;
-} small_class_heap_v4;
+} SmallClassHeap;
 
-// TLS heap context (per-thread)
-typedef struct small_heap_ctx_v4 {
-    small_class_heap_v4 cls[SMALLOBJECT_NUM_CLASSES];
-} small_heap_ctx_v4;
+// SmallHeapCtx: TLS heap context (per-thread)
+typedef struct SmallHeapCtx {
+    SmallClassHeap cls[SMALLOBJECT_NUM_CLASSES];
+} SmallHeapCtx;
 
-// TLS accessor (実装は後続フェーズで追加)
-// SmallPageMeta を external で使う場合のエイリアス
-typedef small_page_v4 SmallPageMeta;
-typedef small_class_heap_v4 SmallClassHeap;
-typedef small_heap_ctx_v4 SmallHeapCtx;
+// Backward compatibility aliases (deprecated, use SmallXxx directly)
+typedef SmallPageMeta small_page_v4;
+typedef SmallClassHeap small_class_heap_v4;
+typedef SmallHeapCtx small_heap_ctx_v4;
 
 small_heap_ctx_v4* small_heap_ctx_v4_get(void);
 

core/box/smallsegment_v4_box.h

@@ -4,23 +4,29 @@
 
 #include <stdint.h>
 
-// Forward declaration for SmallPageMeta
-struct small_page_v4;
+// Forward declaration
+struct SmallPageMeta;
 
+// ============================================================================
 // SmallSegment: class_idx ごとに小さな segment を管理
-// 基本構造: base, page metadata array
-typedef struct small_segment_v4 {
+// ============================================================================
+
+typedef struct SmallSegment {
     uintptr_t base;           // Segment base address
     uint32_t num_pages;       // Number of pages in segment
     uint32_t owner_tid;       // Thread ID that owns this segment
     uint32_t magic;           // Magic number for validation
     // Page metadata array (flexible, actual size depends on SMALL_PAGE_SIZE / SMALL_SEGMENT_SIZE)
-    // struct small_page_v4* page_meta[];
-} small_segment_v4;
+    // struct SmallPageMeta* page_meta[];
+} SmallSegment;
 
-// API: Segment 取得・解放
-small_segment_v4* smallsegment_v4_acquire(int class_idx);
-void smallsegment_v4_release_if_empty(small_segment_v4* seg, void* page, int class_idx);
+// Backward compatibility alias
+typedef SmallSegment small_segment_v4;
 
-// API: ページメタデータの取得
-struct small_page_v4* smallsegment_v4_page_meta_of(small_segment_v4* seg, void* ptr);
+// ============================================================================
+// API: Segment Management
+// ============================================================================
+
+SmallSegment* smallsegment_v4_acquire(int class_idx);
+void smallsegment_v4_release_if_empty(SmallSegment* seg, void* page, int class_idx);
+struct SmallPageMeta* smallsegment_v4_page_meta_of(SmallSegment* seg, void* ptr);