Phase 9: SuperSlab optimization & EMPTY slab recycling (WIP)

Phase 9-1: O(1) SuperSlab lookup optimization
- Created ss_addr_map_box: Hash table (8192 buckets) for O(1) SuperSlab lookup
- Created ss_tls_hint_box: TLS caching layer for SuperSlab hints
- Integrated hash table into registry (init, insert, remove, lookup)
- Modified hak_super_lookup() to use new hash table
- Expected: 50-80 cycles → 10-20 cycles (not verified - SuperSlab disabled by default)

Phase 9-2: EMPTY slab recycling implementation
- Created slab_recycling_box: SLAB_TRY_RECYCLE() macro following Box pattern
- Integrated into remote drain (superslab_slab.c)
- Integrated into TLS SLL drain (tls_sll_drain_box.h) with touched slab tracking
- Observable: Debug tracing via HAKMEM_SLAB_RECYCLE_TRACE
- Updated Makefile: Added new box objects to 3 build targets

Known Issues:
- SuperSlab registry exhaustion still occurs (unregistration not working)
- shared_pool_release_slab() may not be removing from g_super_reg[]
- Needs investigation before Phase 9-2 can be completed

Expected Impact (when fixed):
- Stage 1 hit rate: 0% → 80%
- shared_fail events: 4 → 0
- Kernel overhead: 55% → 15%
- Throughput: 16.5M → 25-30M ops/s (+50-80%)

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

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

core/superslab_slab.c

@@ -4,6 +4,7 @@
 // Date: 2025-11-28
 
 #include "hakmem_tiny_superslab_internal.h"
+#include "box/slab_recycling_box.h"
 
 // ============================================================================
 // Remote Drain (MPSC queue to freelist conversion)
@@ -108,6 +109,10 @@ void _ss_remote_drain_to_freelist_unsafe(SuperSlab* ss, int slab_idx, TinySlabMe
     // Reset remote count after full drain
     atomic_store_explicit(&ss->remote_counts[slab_idx], 0, memory_order_release);
 
+    // Phase 9-2: Try to recycle slab if EMPTY after remote drain
+    // This fixes the bug where EMPTY slabs accumulate and never get returned to freelist
+    SLAB_TRY_RECYCLE(ss, slab_idx, meta);
+
     // Update freelist/nonempty visibility bits
     uint32_t bit = (1u << slab_idx);
     atomic_fetch_or_explicit(&ss->freelist_mask, bit, memory_order_release);