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/tiny_free_fast_v2.inc.h

@@ -177,8 +177,13 @@ static inline int hak_tiny_free_fast_v2(void* ptr) {
                 TinySlabMeta* m = &ss->slabs[sidx];
                 uint8_t meta_cls = m->class_idx;
                 if (meta_cls < TINY_NUM_CLASSES && meta_cls != (uint8_t)class_idx) {
+                    // Phase 26E: Compile-out g_hdr_meta_fast atomic (default OFF)
+#if HAKMEM_HDR_META_FAST_COMPILED
                     static _Atomic uint32_t g_hdr_meta_fast = 0;
                     uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_fast, 1, memory_order_relaxed);
+#else
+                    uint32_t n = 0;  // No-op when compiled out
+#endif
                     if (n < 16) {
                         fprintf(stderr,
                                 "[FREE_FAST_HDR_META_MISMATCH] hdr_cls=%d meta_cls=%u ptr=%p slab_idx=%d ss=%p\n",