Phase 6-2: Promote Front FastLane Free DeDup (default ON)

Results:
- A/B test: +5.18% on Mixed (10-run, clean env)
- Baseline: 46.68M ops/s
- Optimized: 49.10M ops/s
- Improvement: +2.42M ops/s (+5.18%)

Strategy:
- Eliminate duplicate header validation in front_fastlane_try_free()
- Direct call to free_tiny_fast() when dedup enabled
- Single validation path (no redundant checks)

Success factors:
1. Complete duplicate elimination (free path optimization)
2. Free path importance (50% of Mixed workload)
3. Improved execution stability (CV: 1.00% → 0.58%)

Phase 6 cumulative:
- Phase 6-1 FastLane: +11.13%
- Phase 6-2 Free DeDup: +5.18%
- Total: ~+16-17% from baseline (multiplicative effect)

Promotion:
- Default: HAKMEM_FRONT_FASTLANE_FREE_DEDUP=1 (opt-out)
- Added to MIXED_TINYV3_C7_SAFE preset
- Added to C6_HEAVY_LEGACY_POOLV1 preset
- Rollback: HAKMEM_FRONT_FASTLANE_FREE_DEDUP=0

Files modified:
- core/box/front_fastlane_env_box.h: default 0 → 1
- core/bench_profile.h: added to presets
- CURRENT_TASK.md: Phase 6-2 GO result

Health check: PASSED (all profiles)

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

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

core/box/front_fastlane_env_box.h

@@ -39,6 +39,7 @@
 // Forward declaration for cross-box includes
 static inline int front_fastlane_enabled(void);
 static inline uint8_t front_fastlane_class_mask(void);
+static inline int front_fastlane_free_dedup_enabled(void);
 
 // ============================================================================
 // ENV Gate Implementation
@@ -95,4 +96,29 @@ static inline uint8_t front_fastlane_class_mask(void) {
     return (uint8_t)val;
 }
 
+// Phase 6-2: Free DeDup gate (eliminate duplicate header validation)
+// When enabled, front_fastlane_try_free() directly calls free_tiny_fast()
+// instead of doing its own header validation.
+static inline int front_fastlane_free_dedup_enabled(void) {
+    static _Atomic int cached = -1;  // -1 = uninitialized
+    int val = atomic_load_explicit(&cached, memory_order_relaxed);
+
+    if (__builtin_expect(val == -1, 0)) {
+        // Cold path: First call, check ENV
+        const char* env = getenv("HAKMEM_FRONT_FASTLANE_FREE_DEDUP");
+        int enabled = 1;  // default: ON (opt-out, +5.18% proven)
+
+        if (env) {
+            // Parse: "0" or empty = disabled, "1" or non-empty = enabled
+            enabled = (env[0] != '0' && env[0] != '\0') ? 1 : 0;
+        }
+
+        // Cache result (thread-safe: atomic store)
+        atomic_store_explicit(&cached, enabled, memory_order_relaxed);
+        val = enabled;
+    }
+
+    return val;
+}
+
 #endif  // HAK_FRONT_FASTLANE_ENV_BOX_H