Phase v4-mid-6: Implement C6 v4 TLS Fastlist (Gated)

- Implemented TLS fastlist logic for C6 in smallobject_hotbox_v4.c (alloc/free).
- Added SmallC6FastState struct and g_small_c6_fast TLS variable.
- Gated the fastlist logic with HAKMEM_SMALL_HEAP_V4_FASTLIST (default OFF) due to observed instability in mixed workloads.
- Fixed a memory leak in small_heap_free_fast_v4 fallback path by calling hak_pool_free.
- Updated CURRENT_TASK.md with phase report.

core/box/smallobject_hotbox_v4_box.h

@@ -48,6 +48,15 @@ typedef struct SmallHeapCtx {
     SmallClassHeap cls[SMALLOBJECT_NUM_CLASSES];
 } SmallHeapCtx;
 
+// SmallC6FastState: TLS fastlist state for C6 (Phase v4-mid-6)
+typedef struct SmallC6FastState {
+    void* freelist;       // C6 fastlist head
+    void* page_base;      // Current C6 page base address
+    uint32_t used;        // Usage count (synced to page->used on switch)
+    uint32_t capacity;    // Page capacity
+    SmallPageMeta* meta;  // Back-pointer to page metadata
+} SmallC6FastState;
+
 // Backward compatibility aliases (deprecated, use SmallXxx directly)
 typedef SmallPageMeta small_page_v4;
 typedef SmallClassHeap small_class_heap_v4;

core/box/smallobject_hotbox_v4_env_box.h

@@ -51,3 +51,17 @@ static inline int small_heap_v4_c6_enabled(void) {
 static inline int small_heap_v4_c5_enabled(void) {
     return small_heap_v4_class_enabled(5);
 }
+
+static inline int small_heap_v4_fastlist_enabled(void) {
+    static int g_fast = -1;
+    if (__builtin_expect(g_fast == -1, 0)) {
+        const char* e = getenv("HAKMEM_SMALL_HEAP_V4_FASTLIST");
+        if (e && *e) {
+            g_fast = (*e != '0') ? 1 : 0;
+        } else {
+            // Default OFF due to SEGV in random mixed
+            g_fast = 0;
+        }
+    }
+    return g_fast;
+}