Phase 74-3: P0 (FASTAPI) - Free-path branch reduction optimization

Implements FASTAPI pattern to move enabled/init/stats checks outside hot loop:
- Added unified_cache_push_fast() fast-path API (no precondition checks)
- Modified tiny_legacy_fallback_free_base_with_env() to use FASTAPI with ENV gate
- Single boundary point: fallback to slow path if FULL or FASTAPI disabled
- ENV gate: HAKMEM_TINY_UC_FASTAPI=0/1 (default 0, research box)

Results (10-run Mixed SSOT, WS=400):
- Throughput: +0.32% (NEUTRAL, below +1.0% GO threshold)
- cache-misses: -16.31% (positive signal, but throughput gains insufficient)

Status: NEUTRAL, P0 FROZEN (insufficient ROI for structural change)
Next: Phase 74-4 (P2: Hot-class Inline Slots) pending per-class analysis

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

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

core/box/tiny_legacy_fallback_box.h

@@ -11,6 +11,7 @@
 #include "tiny_front_hot_box.h"
 #include "tiny_metadata_cache_env_box.h"  // Phase 3 C2: Metadata cache ENV gate
 #include "hakmem_env_snapshot_box.h"  // Phase 4 E1: ENV snapshot consolidation
+#include "tiny_unified_cache_fastapi_env_box.h"  // Phase 74-3: FASTAPI ENV gate
 
 // Purpose: Encapsulate legacy free logic (shared by multiple paths)
 // Called by: malloc_tiny_fast.h (free path) + tiny_c6_ultra_free_box.c (C6 fallback)
@@ -40,6 +41,27 @@ static inline void tiny_legacy_fallback_free_base_with_env(void* base, uint32_t
 
     // Legacy fallback - Unified Cache push
     if (!front_snap || front_snap->unified_cache_on) {
+        // Phase 74-3 (P0): FASTAPI path (ENV-gated)
+        if (tiny_uc_fastapi_enabled()) {
+            // Preconditions guaranteed:
+            // - unified_cache_on == true (checked above)
+            // - TLS init guaranteed by front_gate_unified_enabled() in malloc_tiny_fast.h
+            // - Stats compiled-out in FAST builds
+            if (unified_cache_push_fast(class_idx, HAK_BASE_FROM_RAW(base))) {
+                FREE_PATH_STAT_INC(legacy_fallback);
+
+                // Per-class breakdown (Phase 4-1)
+                if (__builtin_expect(free_path_stats_enabled(), 0)) {
+                    if (class_idx < 8) {
+                        g_free_path_stats.legacy_by_class[class_idx]++;
+                    }
+                }
+                return;
+            }
+            // FULL → fallback to slow path (rare)
+        }
+
+        // Original path (FASTAPI=0 or fallback)
         if (unified_cache_push(class_idx, HAK_BASE_FROM_RAW(base))) {
             FREE_PATH_STAT_INC(legacy_fallback);