Phase V6-HDR-0: C6-only headerless core 設計確定

- CURRENT_TASK.md: V6-HDR-0 セクション追加（4層 Box Theory）
- SMALLOBJECT_CORE_V6_DESIGN.md: V6-HDR-0 設計方針追加
- REGIONID_V6_DESIGN.md: RegionIdBox 設計書新規作成
- smallobject_core_v6_box.h: SmallTlsLaneV6 型＋TLS API 追加
- smallobject_core_v6.c: OBSERVE モード追加
- region_id_v6_box.h: RegionIdBox 型スケルトン
- page_stats_v6_box.h: PageStatsV6 箱スケルトン
- AGENTS.md: v6 研究箱ルールセクション追加

サニティベンチ: Mixed 42.1M, C6-heavy 25.0M（挙動不変確認）

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

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

core/smallobject_core_v6.c

@@ -1,7 +1,10 @@
-// smallobject_core_v6.c - SmallObject Core v6 実装（Phase v6-3）
+// smallobject_core_v6.c - SmallObject Core v6 実装
+//
+// Phase V6-HDR-0: C6-only headerless core with OBSERVE mode
 
 #include <stdlib.h>
 #include <string.h>
+#include <stdio.h>
 #include "box/smallobject_core_v6_box.h"
 #include "box/smallobject_cold_iface_v6.h"
 #include "box/smallsegment_v6_box.h"
@@ -12,6 +15,31 @@
 #define unlikely(x) __builtin_expect(!!(x), 0)
 #endif
 
+// ============================================================================
+// OBSERVE Mode (V6-HDR-0)
+// ============================================================================
+// ENV: HAKMEM_SMALL_V6_OBSERVE=1 enables logging at free entry
+
+#define V6_OBSERVE_UNINIT  (-1)
+#define V6_OBSERVE_OFF     0
+#define V6_OBSERVE_ON      1
+
+static int g_v6_observe = V6_OBSERVE_UNINIT;
+
+static inline int small_v6_observe_enabled(void) {
+    if (unlikely(g_v6_observe == V6_OBSERVE_UNINIT)) {
+        const char* env = getenv("HAKMEM_SMALL_V6_OBSERVE");
+        g_v6_observe = (env && env[0] == '1') ? V6_OBSERVE_ON : V6_OBSERVE_OFF;
+    }
+    return g_v6_observe == V6_OBSERVE_ON;
+}
+
+/// Log free entry (called when OBSERVE=1)
+static void small_v6_observe_free(void* ptr, uint32_t class_idx, int tls_owned) {
+    fprintf(stderr, "[V6_OBSERVE] free ptr=%p class=%u tls_owned=%d\n",
+            ptr, class_idx, tls_owned);
+}
+
 // TLS context
 static __thread struct SmallHeapCtxV6 g_small_heap_ctx_v6;
 static __thread int g_small_heap_ctx_v6_init = 0;
@@ -250,8 +278,14 @@ void small_free_fast_v6(void* ptr,
     // Convert USER pointer to BASE pointer
     void* base = SMALL_V6_BASE_FROM_USER(ptr);
 
+    // V6-HDR-0: OBSERVE mode logging (check TLS ownership first for log)
+    int tls_owned = small_tls_owns_ptr_v6(ctx, ptr);
+    if (unlikely(small_v6_observe_enabled())) {
+        small_v6_observe_free(ptr, class_idx, tls_owned);
+    }
+
     // Fast path: TLS segment ownership + TLS push
-    if (likely(small_tls_owns_ptr_v6(ctx, ptr))) {
+    if (likely(tls_owned)) {
         // C6 TLS push
         if (class_idx == SMALL_V6_C6_CLASS_IDX && ctx->tls_count_c6 < SMALL_V6_TLS_CAP) {
             ctx->tls_freelist_c6[ctx->tls_count_c6++] = base;