Perf: getenv ホットパスボトルネック削除 (8.51% → 0%)

**問題:**
perf で発見:
- `getenv()`: 8.51% CPU on malloc hot path
- malloc 内で `getenv("HAKMEM_SFC_DEBUG")` が毎回実行
- getenv は環境変数の線形走査 → 非常に重い

**修正内容:**
1. `malloc()`: HAKMEM_SFC_DEBUG を初回のみ getenv して cache (Line 48-52)
2. `malloc()`: HAKMEM_LD_SAFE を初回のみ getenv して cache (Line 75-79)
3. `calloc()`: HAKMEM_LD_SAFE を初回のみ getenv して cache (Line 120-124)

**効果:**
- getenv CPU: 8.51% → 0% ✅
- superslab_refill: 10.30% → 9.61% (-7%)
- hak_tiny_alloc_slow が新トップ: 9.61%

**スループット:**
- 4,192,132 ops/s (変化なし)
- 理由: Syscall Saturation (86.7% kernel time) が支配的
- 次: SuperSlab Caching で syscall 90% 削減 → +100-150% 期待

**Perf結果 (before/after):**
```
Before:  getenv 8.51% | superslab_refill 10.30%
After:   getenv 0%    | hak_tiny_alloc_slow 9.61% | superslab_refill 9.61%
```

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

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

core/box/hak_wrappers.inc.h

@@ -44,8 +44,13 @@ void* malloc(size_t size) {
         return __libc_malloc(size);
     }
 
+    // Cache getenv result to avoid 8.51% CPU overhead on hot path
+    static _Atomic int debug_enabled = -1;  // -1 = uninitialized
     static _Atomic int debug_count = 0;
-    if (getenv("HAKMEM_SFC_DEBUG") && debug_count < 100) {
+    if (__builtin_expect(debug_enabled < 0, 0)) {
+        debug_enabled = (getenv("HAKMEM_SFC_DEBUG") != NULL) ? 1 : 0;
+    }
+    if (debug_enabled && debug_count < 100) {
         int n = atomic_fetch_add(&debug_count, 1);
         if (n < 20) fprintf(stderr, "[SFC_DEBUG] malloc(%zu)\n", size);
     }
@@ -66,9 +71,13 @@ void* malloc(size_t size) {
             extern void* __libc_malloc(size_t);
             return __libc_malloc(size);
         }
+        // Cache HAKMEM_LD_SAFE to avoid repeated getenv on hot path
+        static _Atomic int ld_safe_mode = -1;  // -1 = uninitialized
+        if (__builtin_expect(ld_safe_mode < 0, 0)) {
             const char* lds = getenv("HAKMEM_LD_SAFE");
-        int mode = (lds ? atoi(lds) : 1);
-        if (mode >= 2 || size > TINY_MAX_SIZE) {
+            ld_safe_mode = (lds ? atoi(lds) : 1);
+        }
+        if (ld_safe_mode >= 2 || size > TINY_MAX_SIZE) {
             extern void* __libc_malloc(size_t);
             return __libc_malloc(size);
         }
@@ -106,10 +115,15 @@ void* calloc(size_t nmemb, size_t size) {
         if (hak_ld_block_jemalloc() && hak_jemalloc_loaded()) { extern void* __libc_calloc(size_t, size_t); return __libc_calloc(nmemb, size); }
         if (!g_initialized) { hak_init(); }
         if (g_initializing) { extern void* __libc_calloc(size_t, size_t); return __libc_calloc(nmemb, size); }
+        // Reuse cached ld_safe_mode from malloc (same static variable scope won't work, use inline function instead)
+        // For now, duplicate the caching logic
+        static _Atomic int ld_safe_mode_calloc = -1;
+        if (__builtin_expect(ld_safe_mode_calloc < 0, 0)) {
             const char* lds = getenv("HAKMEM_LD_SAFE");
-        int mode = (lds ? atoi(lds) : 1);
+            ld_safe_mode_calloc = (lds ? atoi(lds) : 1);
+        }
         size_t total = nmemb * size;
-        if (mode >= 2 || total > TINY_MAX_SIZE) { extern void* __libc_calloc(size_t, size_t); return __libc_calloc(nmemb, size); }
+        if (ld_safe_mode_calloc >= 2 || total > TINY_MAX_SIZE) { extern void* __libc_calloc(size_t, size_t); return __libc_calloc(nmemb, size); }
     }
     g_hakmem_lock_depth++;
     size_t total_size = nmemb * size;