Perf optimization: Add __builtin_expect hints to hot paths

Problem: Branch mispredictions in allocation hot paths.
Perf analysis suggested adding likely/unlikely hints.

Solution: Added __builtin_expect hints to critical allocation paths:
1. smallmid_is_enabled() - unlikely (disabled by default)
2. sm_ptr/tiny_ptr/pool_ptr/mid_ptr null checks - likely (success expected)

Optimized locations (core/box/hak_alloc_api.inc.h):
- Line 44: smallmid check (unlikely)
- Line 53: smallmid success check (likely)
- Line 81: tiny success check (likely)
- Line 112: pool success check (likely)
- Line 126: mid success check (likely)

Benchmark results (10M iterations × 5 runs, ws=256):
- Before (Opt2): 71.30M ops/s (avg)
- After (Opt3):  72.92M ops/s (avg)
- Improvement: +2.3% (+1.62M ops/s)

Matches Task agent's prediction of +2-3% throughput gain.

Perf analysis: commit 53bc92842

core/box/hak_alloc_api.inc.h

@@ -40,7 +40,8 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
     // ENV: HAKMEM_SMALLMID_ENABLE=1 to enable (default: OFF)
     // CRITICAL: Must come BEFORE Tiny to avoid routing conflict
     // When enabled, auto-adjusts Tiny to C0-C5 (0-255B only)
-    if (smallmid_is_enabled() && smallmid_is_in_range(size)) {
+    // PERF_OPT: unlikely hint - smallmid disabled by default
+    if (__builtin_expect(smallmid_is_enabled() && smallmid_is_in_range(size), 0)) {
 #if HAKMEM_DEBUG_TIMING
         HKM_TIME_START(t_smallmid);
 #endif
@@ -48,7 +49,8 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
 #if HAKMEM_DEBUG_TIMING
         HKM_TIME_END(HKM_CAT_TINY_ALLOC, t_smallmid);
 #endif
-        if (sm_ptr) {
+        // PERF_OPT: likely hint - smallmid usually succeeds when enabled
+        if (__builtin_expect(sm_ptr != NULL, 1)) {
             hkm_ace_track_alloc();
             return sm_ptr;
         }
@@ -75,7 +77,8 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
 #if HAKMEM_DEBUG_TIMING
         HKM_TIME_END(HKM_CAT_TINY_ALLOC, t_tiny);
 #endif
-        if (tiny_ptr) { hkm_ace_track_alloc(); return tiny_ptr; }
+        // PERF_OPT: likely hint - tiny allocations usually succeed (hot path)
+        if (__builtin_expect(tiny_ptr != NULL, 1)) { hkm_ace_track_alloc(); return tiny_ptr; }
 
         // PHASE 7 CRITICAL FIX: No malloc fallback for Tiny failures
         // If Tiny fails for size <= tiny_get_max_size(), let it flow to Mid/ACE layers
@@ -105,7 +108,8 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
     // Phase 1: Ultra-fast Pool TLS for 8KB-52KB range
     if (size >= 8192 && size <= 53248) {
         void* pool_ptr = pool_alloc(size);
-        if (pool_ptr) return pool_ptr;
+        // PERF_OPT: likely hint - pool allocations usually succeed
+        if (__builtin_expect(pool_ptr != NULL, 1)) return pool_ptr;
         // Fall through to existing Mid allocator as fallback
     }
 #endif
@@ -118,7 +122,8 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
 #if HAKMEM_DEBUG_TIMING
         HKM_TIME_END(HKM_CAT_POOL_GET, t_mid);
 #endif
-        if (mid_ptr) return mid_ptr;
+        // PERF_OPT: likely hint - mid allocations usually succeed
+        if (__builtin_expect(mid_ptr != NULL, 1)) return mid_ptr;
     }
 
 #if HAKMEM_FEATURE_EVOLUTION