Phase 7-Step8: Replace SFC/HEAP_V2/ULTRA_SLIM runtime checks with config macros

**Goal**: Complete dead code elimination infrastructure for all runtime checks

**Changes**:
1. core/box/tiny_front_config_box.h:
   - Rename sfc_cascade_enabled() → tiny_sfc_enabled() (avoid name collision)
   - Update TINY_FRONT_SFC_ENABLED macro to use tiny_sfc_enabled()

2. core/tiny_alloc_fast.inc.h (5 locations):
   - Line 274: tiny_heap_v2_alloc_by_class() - use TINY_FRONT_HEAP_V2_ENABLED
   - Line 431: SFC TLS cache init - use TINY_FRONT_SFC_ENABLED
   - Line 678: SFC cascade check - use TINY_FRONT_SFC_ENABLED
   - Line 740: Ultra SLIM debug check - use TINY_FRONT_ULTRA_SLIM_ENABLED

3. core/hakmem_tiny_free.inc (1 location):
   - Line 233: Heap V2 free path - use TINY_FRONT_HEAP_V2_ENABLED

**Performance**: 79.5M ops/s (maintained, -0.4M vs Step 7, within noise)
- Normal mode: Neutral (runtime checks preserved)
- PGO mode: Ready for dead code elimination

**Total Runtime Checks Replaced (Phase 7)**:
- ✅ TINY_FRONT_FASTCACHE_ENABLED: 3 locations (Step 4-6)
- ✅ TINY_FRONT_TLS_SLL_ENABLED: 7 locations (Step 7)
- ✅ TINY_FRONT_SFC_ENABLED: 2 locations (Step 8)
- ✅ TINY_FRONT_HEAP_V2_ENABLED: 2 locations (Step 8)
- ✅ TINY_FRONT_ULTRA_SLIM_ENABLED: 1 location (Step 8)
**Total**: 15 runtime checks → config macros

**PGO Mode Expected Benefit**:
- Eliminate 15 runtime checks across hot paths
- Reduce branch mispredictions
- Smaller code size (dead code removed by compiler)
- Better instruction cache locality

**Design Complete**: Config Box as single entry point for all Tiny Front policy
- Unified macro interface for all feature toggles
- Include order independent (static inline wrappers)
- Dual-mode support (PGO compile-time vs normal runtime)

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

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

core/tiny_alloc_fast.inc.h

@@ -270,7 +270,8 @@ static inline void* tiny_heap_v2_alloc_by_class(int class_idx) {
     // FIX: Ensure TLS is initialized before first magazine access
     tiny_heap_v2_ensure_init();
     if (class_idx < 0 || class_idx > 3) return NULL;
-    if (!tiny_heap_v2_enabled()) return NULL;
+    // Phase 7-Step8: Use config macro for dead code elimination in PGO mode
+    if (!TINY_FRONT_HEAP_V2_ENABLED) return NULL;
     if (!tiny_heap_v2_class_enabled(class_idx)) return NULL;
 
     TinyHeapV2Mag* mag = &g_tiny_heap_v2_mag[class_idx];
@@ -424,11 +425,11 @@ static inline void* tiny_alloc_fast_pop(int class_idx) {
     }
 
     // Box 5-NEW: Layer 0 - Try SFC first (if enabled)
-    // Cache g_sfc_enabled in TLS to avoid global load on every allocation
+    // Phase 7-Step8: Use config macro for dead code elimination in PGO mode
     static __thread int sfc_check_done = 0;
     static __thread int sfc_is_enabled = 0;
     if (__builtin_expect(!sfc_check_done, 0)) {
-        sfc_is_enabled = g_sfc_enabled;
+        sfc_is_enabled = TINY_FRONT_SFC_ENABLED;
         sfc_check_done = 1;
     }
 
@@ -674,7 +675,8 @@ static inline int tiny_alloc_fast_refill(int class_idx) {
     }
 
     // Only cascade if explicitly enabled AND we have refilled blocks in SLL
-    if (sfc_cascade_enabled && g_sfc_enabled && refilled > 0) {
+    // Phase 7-Step8: Use config macro for dead code elimination in PGO mode
+    if (sfc_cascade_enabled && TINY_FRONT_SFC_ENABLED && refilled > 0) {
         // Transfer half of refilled blocks to SFC (keep half in SLL for future)
         int sfc_target = refilled / 2;
         if (sfc_target > 0) {
@@ -734,7 +736,8 @@ static inline void* tiny_alloc_fast(size_t size) {
 #if !HAKMEM_BUILD_RELEASE
     static __thread int debug_checked = 0;
     if (!debug_checked) {
-        int enabled = ultra_slim_mode_enabled();
+        // Phase 7-Step8: Use config macro for dead code elimination in PGO mode
+        int enabled = TINY_FRONT_ULTRA_SLIM_ENABLED;
         if (enabled) {
             fprintf(stderr, "[TINY_ALLOC_FAST] Ultra SLIM gate: ENABLED (will use 4-layer path)\n");
         } else {