Phase ML1: Pool v1 memset 89.73% overhead 軽量化 (+15.34% improvement)

## Summary
- ChatGPT により bench_profile.h の setenv segfault を修正（RTLD_NEXT 経由に切り替え）
- core/box/pool_zero_mode_box.h 新設：ENV キャッシュ経由で ZERO_MODE を統一管理
- core/hakmem_pool.c で zero mode に応じた memset 制御（FULL/header/off）
- A/B テスト結果：ZERO_MODE=header で +15.34% improvement（1M iterations, C6-heavy）

## Files Modified
- core/box/pool_api.inc.h: pool_zero_mode_box.h include
- core/bench_profile.h: glibc setenv → malloc+putenv（segfault 回避）
- core/hakmem_pool.c: zero mode 参照・制御ロジック
- core/box/pool_zero_mode_box.h (新設): enum/getter
- CURRENT_TASK.md: Phase ML1 結果記載

## Test Results
| Iterations | ZERO_MODE=full | ZERO_MODE=header | Improvement |
|-----------|----------------|-----------------|------------|
| 10K       | 3.06 M ops/s   | 3.17 M ops/s    | +3.65%     |
| 1M        | 23.71 M ops/s  | 27.34 M ops/s   | **+15.34%** |

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

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

core/hakmem_tiny_fastcache.inc.h

@@ -92,9 +92,7 @@ static inline __attribute__((always_inline)) hak_base_ptr_t tiny_fast_pop(int cl
     // Phase 7: header-aware next pointer (C0-C6: base+1, C7: base)
 #if HAKMEM_TINY_HEADER_CLASSIDX
     // Phase E1-CORRECT: ALL classes have 1-byte header, next ptr at offset 1
-    const size_t next_offset = 1;
 #else
-    const size_t next_offset = 0;
 #endif
     // Phase E1-CORRECT: Use Box API for next pointer read (ALL classes: base+1)
     #include "box/tiny_next_ptr_box.h"
@@ -172,9 +170,7 @@ static inline __attribute__((always_inline)) int tiny_fast_push(int class_idx, h
     // Phase 7: header-aware next pointer (C0-C6: base+1, C7: base)
 #if HAKMEM_TINY_HEADER_CLASSIDX
     // Phase E1-CORRECT: ALL classes have 1-byte header, next ptr at offset 1
-    const size_t next_offset2 = 1;
 #else
-    const size_t next_offset2 = 0;
 #endif
     // Phase E1-CORRECT: Use Box API for next pointer write (ALL classes: base+1)
     #include "box/tiny_next_ptr_box.h"