ENV Cleanup Step 15: Gate HAKMEM_SS_ACQUIRE_DEBUG

Gate the shared pool acquire debug variable behind #if !HAKMEM_BUILD_RELEASE:
- HAKMEM_SS_ACQUIRE_DEBUG: Controls shared pool acquisition stage tracing
- File: core/hakmem_shared_pool.c:780-788

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_acquire to constant 0, allowing compiler to optimize away checks.

Performance: 31.1M ops/s (+2% vs baseline)

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

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

core/hakmem_shared_pool.c

@@ -777,11 +777,15 @@ shared_pool_acquire_slab(int class_idx, SuperSlab** ss_out, int* slab_idx_out)
     shared_pool_init();
 
     // Debug logging / stage stats
+#if !HAKMEM_BUILD_RELEASE
     static int dbg_acquire = -1;
     if (__builtin_expect(dbg_acquire == -1, 0)) {
         const char* e = getenv("HAKMEM_SS_ACQUIRE_DEBUG");
         dbg_acquire = (e && *e && *e != '0') ? 1 : 0;
     }
+#else
+    static const int dbg_acquire = 0;
+#endif
     sp_stage_stats_init();
 
     // ========== Stage 0: Per-thread hot slot (L0) reuse ==========