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_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);