Phase v5-0 refactor: ENV統一・マクロ化・構造体最適化

- ENV initialization を sentinel パターンで統一
  - ENV_UNINIT/ENABLED/DISABLED 定数追加
  - __builtin_expect で初期化チェックを最適化
  - small_heap_v5_enabled/class_mask を統一パターンに変更

- ポインタマクロ化（O(1) segment/page 計算）
  - SMALL_SEGMENT_V5_BASE_FROM_PTR: ptr から segment base を mask で計算
  - SMALL_SEGMENT_V5_PAGE_IDX: segment 内の page_idx を shift で計算
  - SMALL_SEGMENT_V5_PAGE_META: page_meta への O(1) access（bounds check付き）
  - SMALL_SEGMENT_V5_VALIDATE_MAGIC: magic 検証
  - SMALL_SEGMENT_V5_VALIDATE_PTR: Fail-Fast validation pipeline

- SmallClassHeapV5 に partial_count 追加
  - partial ページリストのカウンタを追加（refill/retire 最適化用）

- SmallPageMetaV5 の field 再配置（L1 cache 最適化）
  - hot fields (free_list, used, capacity) を先頭に集約
  - metadata (class_idx, flags, page_idx, segment) を後方配置
  - total 24B、offset コメント追加

- route priority ENV 追加
  - HAKMEM_ROUTE_PRIORITY={v4|v5|auto}（default: v4）
  - enum small_route_priority 定義
  - small_route_priority() 関数追加

- segment_size override ENV 追加
  - HAKMEM_SMALL_HEAP_V5_SEGMENT_SIZE（default: 2MiB）
  - power of 2 & >= 64KiB validation

挙動: 完全不変（v5 route は呼ばれない、ENV default OFF）
テスト: Mixed 16–1024B で 43.0–43.8M ops/s（変化なし）、SEGV/assert なし

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

core/box/smallobject_v5_env_box.h

@@ -7,29 +7,42 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
+
+// ENV sentinel values
+#define ENV_UNINIT   (-1)
+#define ENV_ENABLED  (1)
+#define ENV_DISABLED (0)
+
+// route priority enum
+enum small_route_priority {
+    ROUTE_PRIORITY_V4   = 0,
+    ROUTE_PRIORITY_V5   = 1,
+    ROUTE_PRIORITY_AUTO = 2,
+};
 
 // small_heap_v5_enabled() - グローバル v5 enable check
 static inline int small_heap_v5_enabled(void) {
-    static int g_enabled = -1;
-    if (g_enabled == -1) {
+    static int g_enabled = ENV_UNINIT;
+    if (__builtin_expect(g_enabled == ENV_UNINIT, 0)) {
         const char* e = getenv("HAKMEM_SMALL_HEAP_V5_ENABLED");
-        g_enabled = (e && *e && *e != '0') ? 1 : 0;
+        g_enabled = (e && *e && *e != '0') ? ENV_ENABLED : ENV_DISABLED;
     }
-    return g_enabled;
+    return (g_enabled == ENV_ENABLED);
 }
 
 // small_heap_v5_class_mask() - v5 対象クラスのビットマスク
 static inline uint32_t small_heap_v5_class_mask(void) {
-    static uint32_t g_mask = (uint32_t)-1;  // sentinel
-    if (g_mask == (uint32_t)-1) {
+    static int g_mask = ENV_UNINIT;  // sentinel
+    if (__builtin_expect(g_mask == ENV_UNINIT, 0)) {
         const char* e = getenv("HAKMEM_SMALL_HEAP_V5_CLASSES");
         if (e && *e) {
-            g_mask = (uint32_t)strtoul(e, NULL, 0);
+            g_mask = (int)strtoul(e, NULL, 0);
         } else {
             g_mask = 0x0;  // default: OFF
         }
     }
-    return g_mask;
+    return (uint32_t)g_mask;
 }
 
 // small_heap_v5_class_enabled() - 指定クラスが v5 有効か
@@ -53,4 +66,46 @@ static inline int small_heap_v5_c7_enabled(void) {
     return small_heap_v5_class_enabled(7);
 }
 
+// small_route_priority() - route priority (v4/v5/auto)
+// ENV: HAKMEM_ROUTE_PRIORITY={v4|v5|auto}, default: v4
+static inline int small_route_priority(void) {
+    static int g_priority = ENV_UNINIT;
+    if (__builtin_expect(g_priority == ENV_UNINIT, 0)) {
+        const char* e = getenv("HAKMEM_ROUTE_PRIORITY");
+        if (e && *e) {
+            if (strcmp(e, "v5") == 0) {
+                g_priority = ROUTE_PRIORITY_V5;
+            } else if (strcmp(e, "auto") == 0) {
+                g_priority = ROUTE_PRIORITY_AUTO;
+            } else {
+                g_priority = ROUTE_PRIORITY_V4;  // default or "v4"
+            }
+        } else {
+            g_priority = ROUTE_PRIORITY_V4;  // default
+        }
+    }
+    return g_priority;
+}
+
+// small_heap_v5_segment_size() - segment size override
+// ENV: HAKMEM_SMALL_HEAP_V5_SEGMENT_SIZE, default: 2MiB (2*1024*1024)
+static inline size_t small_heap_v5_segment_size(void) {
+    static int g_size = ENV_UNINIT;
+    if (__builtin_expect(g_size == ENV_UNINIT, 0)) {
+        const char* e = getenv("HAKMEM_SMALL_HEAP_V5_SEGMENT_SIZE");
+        if (e && *e) {
+            size_t sz = (size_t)strtoul(e, NULL, 0);
+            // validate: must be power of 2, >= 64KiB
+            if (sz >= (64 * 1024) && (sz & (sz - 1)) == 0) {
+                g_size = (int)sz;
+            } else {
+                g_size = (2 * 1024 * 1024);  // fallback to default
+            }
+        } else {
+            g_size = (2 * 1024 * 1024);  // default: 2MiB
+        }
+    }
+    return (size_t)g_size;
+}
+
 #endif // HAKMEM_SMALLOBJECT_V5_ENV_BOX_H