Phase 70-0: Infrastructure prep for refill tuning (Observability + WarmPool Sizing)

- Observability Fix: Enabled unified cache hit/miss stats in release builds when HAKMEM_UNIFIED_CACHE_STATS_COMPILED is set.
- WarmPool Sizing: Decoupled hardcoded '12' from prefill logic; now uses TINY_WARM_POOL_DEFAULT_PER_CLASS macro and respects ENV overrides.
- Increased TINY_WARM_POOL_MAX_PER_CLASS to 32 to support wider ENV sweeps.
- Added unified_cache_auto_stats destructor to dump metrics at exit (replacing debug print hack).

core/hakmem_shared_pool_acquire.c

@@ -266,6 +266,9 @@ sp_acquire_from_empty_scan(int class_idx, SuperSlab** ss_out, int* slab_idx_out,
     SuperSlab* primary_result = NULL;
     int primary_slab_idx = -1;
 
+    // Cache warm pool cap once per acquire call (SSOT: same as unified_cache_refill()).
+    const int warm_cap = warm_pool_max_per_class();
+
     for (int i = 0; i < scan_limit; i++) {
         SuperSlab* ss = super_reg_by_class_at(class_idx, i);
         if (!(ss && ss->magic == SUPERSLAB_MAGIC)) continue;
@@ -274,9 +277,8 @@ sp_acquire_from_empty_scan(int class_idx, SuperSlab** ss_out, int* slab_idx_out,
         if (ss->empty_count == 0) continue;  // No EMPTY slabs in this SS
 
         // WARM POOL PREFILL: Add HOT SuperSlabs to warm pool (if not already primary result)
-        // This is low-cost during registry scan and avoids future expensive scans
-        // Phase 1: Increase threshold from 4 to 12 to match TINY_WARM_POOL_MAX_PER_CLASS
-        if (ss != primary_result && tiny_warm_pool_count(class_idx) < 12) {
+        // This is low-cost during registry scan and avoids future expensive scans.
+        if (ss != primary_result && tiny_warm_pool_count(class_idx) < warm_cap) {
             tiny_warm_pool_push(class_idx, ss);
             // Track prefilled SuperSlabs for metrics
             g_warm_pool_stats[class_idx].prefilled++;