ENV cleanup: Add RELEASE guards to DEBUG ENV variables (14 vars)

Added compile-time guards (#if HAKMEM_BUILD_RELEASE) to eliminate
DEBUG ENV variable overhead in RELEASE builds.

Variables guarded (14 total):
- HAKMEM_TINY_TRACE_RING, HAKMEM_TINY_DUMP_RING_ATEXIT
- HAKMEM_TINY_RF_TRACE, HAKMEM_TINY_MAILBOX_TRACE
- HAKMEM_TINY_MAILBOX_TRACE_LIMIT, HAKMEM_TINY_MAILBOX_SLOWDISC
- HAKMEM_TINY_MAILBOX_SLOWDISC_PERIOD
- HAKMEM_SS_PREWARM_DEBUG, HAKMEM_SS_FREE_DEBUG
- HAKMEM_TINY_FRONT_METRICS, HAKMEM_TINY_FRONT_DUMP
- HAKMEM_TINY_COUNTERS_DUMP, HAKMEM_TINY_REFILL_DUMP
- HAKMEM_PTR_TRACE_DUMP, HAKMEM_PTR_TRACE_VERBOSE

Files modified (9 core files):
- core/tiny_debug_ring.c (ring trace/dump)
- core/box/mailbox_box.c (mailbox trace + slowdisc)
- core/tiny_refill.h (refill trace)
- core/hakmem_tiny_superslab.c (superslab debug)
- core/box/ss_allocation_box.c (allocation debug)
- core/tiny_superslab_free.inc.h (free debug)
- core/box/front_metrics_box.c (frontend metrics)
- core/hakmem_tiny_stats.c (stats dump)
- core/ptr_trace.h (pointer trace)

Bug fixes during implementation:
1. mailbox_box.c - Fixed variable scope (moved 'used' outside guard)
2. hakmem_tiny_stats.c - Fixed incomplete declarations (on1, on2)

Impact:
- Binary size: -85KB total
  - bench_random_mixed_hakmem: 319K → 305K (-14K, -4.4%)
  - larson_hakmem: 380K → 309K (-71K, -18.7%)
- Performance: No regression (16.9-17.9M ops/s maintained)
- Functional: All tests pass (Random Mixed + Larson)
- Behavior: DEBUG ENV vars correctly ignored in RELEASE builds

Testing:
- Build: Clean compilation (warnings only, pre-existing)
- 100K Random Mixed: 16.9-17.9M ops/s (PASS)
- 10K Larson: 25.9M ops/s (PASS)
- DEBUG ENV verification: Correctly ignored (PASS)

Result: 14 DEBUG ENV variables now have zero overhead in RELEASE builds.

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

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

core/tiny_debug_ring.c

@@ -191,6 +191,9 @@ static void tiny_debug_ring_sigusr(int signo, siginfo_t* info, void* uctx) {
 }
 
 void tiny_debug_ring_init(void) {
+#if HAKMEM_BUILD_RELEASE
+    return;  // No env reads or hooks in release builds
+#endif
     if (g_tiny_ring_enabled) return;
     const char* env = getenv("HAKMEM_TINY_TRACE_RING");
     if (!(env && *env && env[0] != '0')) {
@@ -228,6 +231,9 @@ static void tiny_debug_ring_ctor(void) {
 
 __attribute__((destructor))
 static void tiny_debug_ring_dtor(void) {
+#if HAKMEM_BUILD_RELEASE
+    return;  // Skip env access in release builds
+#endif
     const char* e = getenv("HAKMEM_TINY_DUMP_RING_ATEXIT");
     if (e && *e && e[0] != '0') {
         tiny_debug_ring_dump(STDERR_FILENO, 0);