Phase 23 Unified Cache + PageFaultTelemetry generalization: Mid/VM page-fault bottleneck identified

Summary:
- Phase 23 Unified Cache: +30% improvement (Random Mixed 256B: 18.18M → 23.68M ops/s)
- PageFaultTelemetry: Extended to generic buckets (C0-C7, MID, L25, SSM)
- Measurement-driven decision: Mid/VM page-faults (80-100K) >> Tiny (6K) → prioritize Mid/VM optimization

Phase 23 Changes:
1. Unified Cache implementation (core/front/tiny_unified_cache.{c,h})
   - Direct SuperSlab carve (TLS SLL bypass)
   - Self-contained pop-or-refill pattern
   - ENV: HAKMEM_TINY_UNIFIED_CACHE=1, HAKMEM_TINY_UNIFIED_C{0-7}=128

2. Fast path pruning (tiny_alloc_fast.inc.h, tiny_free_fast_v2.inc.h)
   - Unified ON → direct cache access (skip all intermediate layers)
   - Alloc: unified_cache_pop_or_refill() → immediate fail to slow
   - Free: unified_cache_push() → fallback to SLL only if full

PageFaultTelemetry Changes:
3. Generic bucket architecture (core/box/pagefault_telemetry_box.{c,h})
   - PF_BUCKET_{C0-C7, MID, L25, SSM} for domain-specific measurement
   - Integration: hak_pool_try_alloc(), l25_alloc_new_run(), shared_pool_allocate_superslab_unlocked()

4. Measurement results (Random Mixed 500K / 256B):
   - Tiny C2-C7: 2-33 pages, high reuse (64-3.8 touches/page)
   - SSM: 512 pages (initialization footprint)
   - MID/L25: 0 (unused in this workload)
   - Mid/Large VM benchmarks: 80-100K page-faults (13-16x higher than Tiny)

Ring Cache Enhancements:
5. Hot Ring Cache (core/front/tiny_ring_cache.{c,h})
   - ENV: HAKMEM_TINY_HOT_RING_ENABLE=1, HAKMEM_TINY_HOT_RING_C{0-7}=size
   - Conditional compilation cleanup

Documentation:
6. Analysis reports
   - RANDOM_MIXED_BOTTLENECK_ANALYSIS.md: Page-fault breakdown
   - RANDOM_MIXED_SUMMARY.md: Phase 23 summary
   - RING_CACHE_ACTIVATION_GUIDE.md: Ring cache usage
   - CURRENT_TASK.md: Updated with Phase 23 results and Phase 24 plan

Next Steps (Phase 24):
- Target: Mid/VM PageArena/HotSpanBox (page-fault reduction 80-100K → 30-40K)
- Tiny SSM optimization deferred (low ROI, ~6K page-faults already optimal)
- Expected improvement: +30-50% for Mid/Large workloads

Generated with Claude Code

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

core/front/tiny_ring_cache.c

@@ -10,6 +10,7 @@
 
 __thread TinyRingCache g_ring_cache_c2 = {NULL, 0, 0, 0, 0};
 __thread TinyRingCache g_ring_cache_c3 = {NULL, 0, 0, 0, 0};
+__thread TinyRingCache g_ring_cache_c5 = {NULL, 0, 0, 0, 0};
 
 // ============================================================================
 // Metrics (Phase 21-1-E, optional for Phase 21-1-C)
@@ -63,10 +64,31 @@ void ring_cache_init(void) {
     g_ring_cache_c3.head = 0;
     g_ring_cache_c3.tail = 0;
 
+    // C5 init
+    size_t cap_c5 = ring_capacity_c5();
+    g_ring_cache_c5.slots = (void**)calloc(cap_c5, sizeof(void*));
+    if (!g_ring_cache_c5.slots) {
 #if !HAKMEM_BUILD_RELEASE
-    fprintf(stderr, "[Ring-INIT] C2=%zu slots (%zu bytes), C3=%zu slots (%zu bytes)\n",
+        fprintf(stderr, "[Ring-INIT] Failed to allocate C5 ring (%zu slots)\n", cap_c5);
+        fflush(stderr);
+#endif
+        // Free C2 and C3 if C5 failed
+        free(g_ring_cache_c2.slots);
+        g_ring_cache_c2.slots = NULL;
+        free(g_ring_cache_c3.slots);
+        g_ring_cache_c3.slots = NULL;
+        return;
+    }
+    g_ring_cache_c5.capacity = (uint16_t)cap_c5;
+    g_ring_cache_c5.mask = (uint16_t)(cap_c5 - 1);
+    g_ring_cache_c5.head = 0;
+    g_ring_cache_c5.tail = 0;
+
+#if !HAKMEM_BUILD_RELEASE
+    fprintf(stderr, "[Ring-INIT] C2=%zu slots (%zu bytes), C3=%zu slots (%zu bytes), C5=%zu slots (%zu bytes)\n",
             cap_c2, cap_c2 * sizeof(void*),
-            cap_c3, cap_c3 * sizeof(void*));
+            cap_c3, cap_c3 * sizeof(void*),
+            cap_c5, cap_c5 * sizeof(void*));
     fflush(stderr);
 #endif
 }
@@ -92,8 +114,13 @@ void ring_cache_shutdown(void) {
         g_ring_cache_c3.slots = NULL;
     }
 
+    if (g_ring_cache_c5.slots) {
+        free(g_ring_cache_c5.slots);
+        g_ring_cache_c5.slots = NULL;
+    }
+
 #if !HAKMEM_BUILD_RELEASE
-    fprintf(stderr, "[Ring-SHUTDOWN] C2/C3 rings freed\n");
+    fprintf(stderr, "[Ring-SHUTDOWN] C2/C3/C5 rings freed\n");
     fflush(stderr);
 #endif
 }