Fix: 500K iteration SEGV - node pool exhaustion + deadlock

Root cause analysis (via Task agent investigation):
- Node pool (512 nodes/class) exhausts at ~500K iterations
- Two separate issues identified:
  1. Deadlock in sp_freelist_push_lockfree (FREE path)
  2. Node pool exhaustion triggering stack corruption (ALLOC path)

Fixes applied:
1. Deadlock fix (core/hakmem_shared_pool.c:382-387):
   - Removed recursive pthread_mutex_lock/unlock in fallback path
   - Caller (shared_pool_release_slab:772) already holds lock
   - Prevents deadlock on non-recursive mutex

2. Node pool expansion (core/hakmem_shared_pool.h:77):
   - Increased MAX_FREE_NODES_PER_CLASS from 512 to 4096
   - Supports 500K+ iterations without exhaustion
   - Prevents stack corruption in hak_tiny_alloc_slow()

Test results:
- Before: SEGV at 500K with "Node pool exhausted for class 7"
- After:  9.44M ops/s, stable, no warnings, no crashes

Note: This fixes Mid-Large allocator's SP-SLOT Box, not Phase B C23 code.
Phase B (TinyFrontC23Box) remains stable and unaffected.

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

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

core/hakmem_shared_pool.h

@@ -74,7 +74,7 @@ typedef struct {
 } LockFreeFreeList;
 
 // Node pool for lock-free allocation (avoid malloc/free)
-#define MAX_FREE_NODES_PER_CLASS 512  // Pre-allocated nodes per class
+#define MAX_FREE_NODES_PER_CLASS 4096  // Pre-allocated nodes per class (increased for 500K+ iterations)
 extern FreeSlotNode g_free_node_pool[TINY_NUM_CLASSES_SS][MAX_FREE_NODES_PER_CLASS];
 extern _Atomic uint32_t g_node_alloc_index[TINY_NUM_CLASSES_SS];