Phase 75-1: C6-only Inline Slots (P2) - GO (+2.87%)

Modular implementation of hot-class inline slots optimization:
- Created 5 new boxes: env_box, tls_box, fast_path_api, integration_box, test_script
- Single decision point at TLS init (ENV gate: HAKMEM_TINY_C6_INLINE_SLOTS=0/1)
- Integration: 2 minimal boundary points (alloc/free paths for C6 class)
- Default OFF: zero overhead when disabled (full backward compatibility)

Results (10-run Mixed SSOT, WS=400):
- Baseline (C6 inline OFF):  44.24 M ops/s
- Treatment (C6 inline ON):  45.51 M ops/s
- Delta: +1.27 M ops/s (+2.87%)

Status: ✅ GO - Strong improvement via C6 ring buffer fast-path
Mechanism: Branch elimination on unified_cache_push/pop for C6 allocations
Next: Phase 75-2 (add C5 inline slots, target 85% C4-C7 coverage)

🤖 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

@@ -12,6 +12,8 @@
 #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
+#include "tiny_c6_inline_slots_env_box.h" // Phase 75-1: C6 inline slots ENV gate
+#include "../front/tiny_c6_inline_slots.h" // Phase 75-1: C6 inline slots API
 
 // 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)
@@ -23,6 +25,20 @@
 //
 __attribute__((always_inline))
 static inline void tiny_legacy_fallback_free_base_with_env(void* base, uint32_t class_idx, const HakmemEnvSnapshot* env) {
+    // Phase 75-1: C6 Inline Slots early-exit (ENV gated)
+    // Try C6 inline slots FIRST (before unified cache) for class 6
+    if (class_idx == 6 && tiny_c6_inline_slots_enabled()) {
+        if (c6_inline_push(c6_inline_tls(), base)) {
+            // Success: pushed to C6 inline slots
+            FREE_PATH_STAT_INC(legacy_fallback);
+            if (__builtin_expect(free_path_stats_enabled(), 0)) {
+                g_free_path_stats.legacy_by_class[class_idx]++;
+            }
+            return;
+        }
+        // FULL → fall through to unified cache
+    }
+
     const TinyFrontV3Snapshot* front_snap =
         env ? (env->tiny_front_v3_enabled ? tiny_front_v3_snapshot_get() : NULL)
             : (__builtin_expect(tiny_front_v3_enabled(), 0) ? tiny_front_v3_snapshot_get() : NULL);