Phase 4d: Add master stats control (HAKMEM_STATS)

Add unified stats/dump control that allows enabling specific stats
modules using comma-separated values or "all" to enable everything.

New file: core/hakmem_stats_master.h
- HAKMEM_STATS=all: Enable all stats modules
- HAKMEM_STATS=sfc,fast,pool: Enable specific modules
- HAKMEM_STATS_DUMP=1: Dump stats at exit
- hak_stats_check(): Check if module should enable stats

Available stats modules:
  sfc, fast, heap, refill, counters, ring, invariant,
  pagefault, front, pool, slim, guard, nearempty

Updated files:
- core/hakmem_tiny_sfc.c: Use hak_stats_check() for SFC stats
- core/hakmem_shared_pool.c: Use hak_stats_check() for pool stats

Performance: No regression (72.9M ops/s)

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

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

core/hakmem_shared_pool.c

@@ -2,6 +2,7 @@
 #include "hakmem_tiny_superslab.h"
 #include "hakmem_tiny_superslab_constants.h"
 #include "hakmem_debug_master.h"  // Phase 4b: Master debug control
+#include "hakmem_stats_master.h"  // Phase 4d: Master stats control
 #include "box/ss_slab_meta_box.h"  // Phase 3d-A: SlabMeta Box boundary
 #include "box/ss_hot_cold_box.h"   // Phase 12-1.1: EMPTY slab marking
 #include "box/pagefault_telemetry_box.h"  // Box PageFaultTelemetry (PF_BUCKET_SS_META)
@@ -80,9 +81,9 @@ static int g_sp_stage_stats_enabled = 0;
 static int g_sp_stage_stats_log_enabled = -1;  // -1=uninitialized, 0=off, 1=on
 
 static inline void sp_stage_stats_init(void) {
+    // Phase 4d: Now uses hak_stats_check() for unified stats control
     if (__builtin_expect(g_sp_stage_stats_log_enabled == -1, 0)) {
-        const char* env = getenv("HAKMEM_SHARED_POOL_STAGE_STATS");
-        g_sp_stage_stats_log_enabled = (env && *env && *env != '0') ? 1 : 0;
+        g_sp_stage_stats_log_enabled = hak_stats_check("HAKMEM_SHARED_POOL_STAGE_STATS", "pool");
         if (g_sp_stage_stats_log_enabled == 1) {
             // ログが有効なら計測も必ず有効化する。
             g_sp_stage_stats_enabled = 1;