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/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);