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/hakmem_shared_pool.c

@@ -1,6 +1,7 @@
 #include "hakmem_shared_pool.h"
 #include "hakmem_tiny_superslab.h"
 #include "hakmem_tiny_superslab_constants.h"
+#include "box/pagefault_telemetry_box.h"  // Box PageFaultTelemetry (PF_BUCKET_SS_META)
 
 #include <stdlib.h>
 #include <string.h>
@@ -477,6 +478,12 @@ shared_pool_allocate_superslab_unlocked(void)
         return NULL;
     }
 
+    // PageFaultTelemetry: mark all backing pages for this Superslab (approximate)
+    size_t ss_bytes = (size_t)1 << ss->lg_size;
+    for (size_t off = 0; off < ss_bytes; off += 4096) {
+        pagefault_telemetry_touch(PF_BUCKET_SS_META, (char*)ss + off);
+    }
+
     // superslab_allocate() already:
     //  - zeroes slab metadata / remote queues,
     //  - sets magic/lg_size/etc,