Phase 24-26: Hot path atomic telemetry prune (+2.00% cumulative)

Summary:
- Phase 24 (alloc stats): +0.93% GO
- Phase 25 (free stats): +1.07% GO
- Phase 26 (diagnostics): -0.33% NEUTRAL (code cleanliness)
- Total: 11 atomics compiled-out, +2.00% improvement

Phase 24: OBSERVE tax prune (tiny_class_stats_box.h)
- Added HAKMEM_TINY_CLASS_STATS_COMPILED (default: 0)
- Wrapped 5 stats functions: uc_miss, warm_hit, shared_lock, tls_carve_*
- Result: +0.93% (baseline 56.675M vs compiled-in 56.151M ops/s)

Phase 25: Tiny free stats prune (tiny_superslab_free.inc.h)
- Added HAKMEM_TINY_FREE_STATS_COMPILED (default: 0)
- Wrapped g_free_ss_enter atomic in free hot path
- Result: +1.07% (baseline 57.017M vs compiled-in 56.415M ops/s)

Phase 26: Hot path diagnostic atomics prune
- Added 5 compile gates for low-frequency error counters:
  - HAKMEM_TINY_C7_FREE_COUNT_COMPILED
  - HAKMEM_TINY_HDR_MISMATCH_LOG_COMPILED
  - HAKMEM_TINY_HDR_META_MISMATCH_COMPILED
  - HAKMEM_TINY_METRIC_BAD_CLASS_COMPILED
  - HAKMEM_TINY_HDR_META_FAST_COMPILED
- Result: -0.33% NEUTRAL (within noise, kept for cleanliness)

Alignment with mimalloc principles:
- "No atomics on hot path" - telemetry moved to compile-time opt-in
- Fixed per-op tax elimination
- Production builds: maximum performance (atomics compiled-out)
- Research builds: full diagnostics (COMPILED=1)

Generated with Claude Code
https://claude.com/claude-code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

core/box/tiny_header_box.h

@@ -212,13 +212,16 @@ void* tiny_region_id_write_header(void* base, int class_idx);
 
 static inline void* tiny_header_finalize_alloc(void* base, int class_idx) {
 #if HAKMEM_TINY_HEADER_CLASSIDX
-    // Write-once optimization: Skip header write for C1-C6 if already prefilled
-    if (tiny_header_write_once_enabled() && tiny_class_preserves_header(class_idx)) {
+#if HAKMEM_TINY_HEADER_WRITE_ONCE_COMPILED
+    // Phase 23: Write-once optimization (compile-out when disabled, default OFF)
+    // Evaluate class check first (short-circuit), then ENV check
+    if (tiny_class_preserves_header(class_idx) && tiny_header_write_once_enabled()) {
         // Header already written at refill boundary → skip write, return USER pointer
         return (void*)((uint8_t*)base + 1);
     }
+#endif
 
-    // Traditional path: C0, C7, or WRITE_ONCE=0
+    // Traditional path: C0, C7, or WRITE_ONCE compiled-out/disabled
     return tiny_region_id_write_header(base, class_idx);
 #else
     (void)class_idx;