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_superslab_free.inc.h

@@ -7,6 +7,7 @@
 // - hak_tiny_free_superslab(): Main SuperSlab free entry point
 
 #include <stdatomic.h>
+#include "hakmem_build_flags.h"  // Phase 25: Compile-time feature switches
 #include "box/ptr_type_box.h"  // Phase 10
 #include "box/free_remote_box.h"
 #include "box/free_local_box.h"
@@ -15,8 +16,13 @@
 // Phase 6.22-B: SuperSlab fast free path
 static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
     // Route trace: count SuperSlab free entries (diagnostics only)
+    // Phase 25: Compile-out free stats atomic (default OFF)
+#if HAKMEM_TINY_FREE_STATS_COMPILED
     extern _Atomic uint64_t g_free_ss_enter;
     atomic_fetch_add_explicit(&g_free_ss_enter, 1, memory_order_relaxed);
+#else
+    (void)0;  // No-op when compiled out
+#endif
     ROUTE_MARK(16); // free_enter
     HAK_DBG_INC(g_superslab_free_count);  // Phase 7.6: Track SuperSlab frees
 
@@ -40,7 +46,9 @@ static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
     uint8_t cls = meta->class_idx;
     
     // Debug: Log first C7 alloc/free for path verification
+    // Phase 26A: Compile-out c7_free_count atomic (default OFF)
     if (cls == 7) {
+#if HAKMEM_C7_FREE_COUNT_COMPILED
         static _Atomic int c7_free_count = 0;
         int count = atomic_fetch_add_explicit(&c7_free_count, 1, memory_order_relaxed);
         if (count == 0) {
@@ -48,6 +56,10 @@ static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
             fprintf(stderr, "[C7_FIRST_FREE] ptr=%p base=%p slab_idx=%d\n", ptr, base, slab_idx);
             #endif
         }
+#else
+        // No-op when compiled out (Phase 26A)
+        (void)0;
+#endif
     }
     if (__builtin_expect(tiny_remote_watch_is(ptr), 0)) {
         tiny_remote_watch_note("free_enter", ss, slab_idx, ptr, 0xA240u, tiny_self_u32(), 0);
@@ -137,8 +149,13 @@ static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
             uint8_t hdr = *(uint8_t*)base;
             uint8_t expect = (uint8_t)(HEADER_MAGIC | (cls & HEADER_CLASS_MASK));
             if (__builtin_expect(hdr != expect, 0)) {
+                // Phase 26B: Compile-out g_hdr_mismatch_log atomic (default OFF)
+#if HAKMEM_HDR_MISMATCH_LOG_COMPILED
                 static _Atomic uint32_t g_hdr_mismatch_log = 0;
                 uint32_t n = atomic_fetch_add_explicit(&g_hdr_mismatch_log, 1, memory_order_relaxed);
+#else
+                uint32_t n = 0;  // No-op when compiled out
+#endif
                 if (n < 8) {
                     fprintf(stderr,
                             "[TLS_HDR_MISMATCH] cls=%u slab_idx=%d hdr=0x%02x expect=0x%02x ptr=%p\n",
@@ -172,8 +189,13 @@ static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
             uint8_t hdr_cls = tiny_region_id_read_header(ptr);
             uint8_t meta_cls = meta->class_idx;
             if (__builtin_expect(hdr_cls != meta_cls, 0)) {
+                // Phase 26C: Compile-out g_hdr_meta_mismatch atomic (default OFF)
+#if HAKMEM_HDR_META_MISMATCH_COMPILED
                 static _Atomic uint32_t g_hdr_meta_mismatch = 0;
                 uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_mismatch, 1, memory_order_relaxed);
+#else
+                uint32_t n = 0;  // No-op when compiled out
+#endif
                 if (n < 16) {
                     fprintf(stderr, "[SLAB_HDR_META_MISMATCH] slab_push cls_meta=%u hdr_cls=%u ptr=%p slab_idx=%d ss=%p freelist=%p used=%u\n",
                             (unsigned)meta_cls, (unsigned)hdr_cls, ptr, slab_idx, (void*)ss, meta->freelist, (unsigned)meta->used);