ENV Cleanup Step 6: Gate HAKMEM_TIMING in core/hakmem_debug.c

Changes:
- Gated HAKMEM_TIMING ENV check behind #if !HAKMEM_BUILD_RELEASE
- Release builds set g_timing_enabled = 0 directly (no getenv call)
- Debug builds preserve existing behavior
- ENV variable gated: HAKMEM_TIMING

Performance: 30.3M ops/s Larson (baseline 30.4M, within margin)
Build: Clean, LTO warnings only (pre-existing)

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

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

core/hakmem_debug.c

@@ -149,11 +149,15 @@ static void atexit_handler(void) {
 void hkm_timing_init(void) {
     if (g_initialized) return;
 
-    // Check HAKMEM_TIMING environment variable
+#if !HAKMEM_BUILD_RELEASE
+    // Check HAKMEM_TIMING environment variable (gated behind !HAKMEM_BUILD_RELEASE)
     const char* env = getenv("HAKMEM_TIMING");
     if (env && strcmp(env, "1") == 0) {
         g_timing_enabled = 1;
     }
+#else
+    g_timing_enabled = 0;  // Always disabled in release builds
+#endif
 
     // Initialize global stats
     memset(g_global_stats, 0, sizeof(g_global_stats));