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/front/tiny_unified_cache.h

@@ -294,6 +294,39 @@ static inline int unified_cache_push(int class_idx, hak_base_ptr_t base) {
 #endif  // HAKMEM_TINY_UC_LOCALIZE_COMPILED
 }
 
+// ============================================================================
+// Phase 74-3 (P0): FASTAPI - Fast-path push (assumes preconditions met)
+// ============================================================================
+// Preconditions (caller must ensure):
+//   - Unified cache is enabled (TINY_FRONT_UNIFIED_CACHE_ENABLED == 1)
+//   - TLS cache is initialized (cache->slots != NULL)
+//   - Stats are compiled-out or caller doesn't need them
+// Returns: 1=SUCCESS, 0=FULL (same as unified_cache_push)
+static inline int unified_cache_push_fast(int class_idx, hak_base_ptr_t base) {
+    void* base_raw = HAK_BASE_TO_RAW(base);
+    TinyUnifiedCache* cache = &g_unified_cache[class_idx];
+
+    uint16_t next_tail = (cache->tail + 1) & cache->mask;
+
+    // Full check (leave 1 slot empty to distinguish full/empty)
+    if (__builtin_expect(next_tail == cache->head, 0)) {
+#if !HAKMEM_BUILD_RELEASE || HAKMEM_UNIFIED_CACHE_STATS_COMPILED
+        g_unified_cache_full[class_idx]++;
+#endif
+        return 0;  // Full
+    }
+
+    // Push to tail (producer)
+    cache->slots[cache->tail] = base_raw;
+    cache->tail = next_tail;
+
+#if !HAKMEM_BUILD_RELEASE || HAKMEM_UNIFIED_CACHE_STATS_COMPILED
+    g_unified_cache_push[class_idx]++;
+#endif
+
+    return 1;  // SUCCESS (FASTAPI path)
+}
+
 // ============================================================================
 // Phase 23-D: Self-Contained Pop-or-Refill (tcache-style, single-layer)
 // ============================================================================