ENV Cleanup Step 16: Gate HAKMEM_SS_FREE_DEBUG

Gate the shared pool free debug variable behind #if !HAKMEM_BUILD_RELEASE:
- HAKMEM_SS_FREE_DEBUG: Controls shared pool slot release tracing
- File: core/hakmem_shared_pool.c:1221-1229

The debug output was already gated inside #if !HAKMEM_BUILD_RELEASE blocks.
This change only gates the ENV check itself. In release builds, sets
dbg to constant 0, allowing compiler to optimize away checks.

Performance: 30.3M ops/s (baseline maintained)

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

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

core/hakmem_shared_pool.c

@@ -1218,11 +1218,15 @@ shared_pool_release_slab(SuperSlab* ss, int slab_idx)
     }
 
     // Debug logging
+#if !HAKMEM_BUILD_RELEASE
     static int dbg = -1;
     if (__builtin_expect(dbg == -1, 0)) {
         const char* e = getenv("HAKMEM_SS_FREE_DEBUG");
         dbg = (e && *e && *e != '0') ? 1 : 0;
     }
+#else
+    static const int dbg = 0;
+#endif
 
     // P0 instrumentation: count lock acquisitions
     lock_stats_init();