P0 Optimization: Shared Pool fast path with O(1) metadata lookup

Performance Results:
- Throughput: 2.66M ops/s → 3.8M ops/s (+43% improvement)
- sp_meta_find_or_create: O(N) linear scan → O(1) direct pointer
- Stage 2 metadata scan: 100% → 10-20% (80-90% reduction via hints)

Core Optimizations:

1. O(1) Metadata Lookup (superslab_types.h)
   - Added `shared_meta` pointer field to SuperSlab struct
   - Eliminates O(N) linear search through ss_metadata[] array
   - First access: O(N) scan + cache | Subsequent: O(1) direct return

2. sp_meta_find_or_create Fast Path (hakmem_shared_pool.c)
   - Check cached ss->shared_meta first before linear scan
   - Cache pointer after successful linear scan for future lookups
   - Reduces 7.8% CPU hotspot to near-zero for hot paths

3. Stage 2 Class Hints Fast Path (hakmem_shared_pool_acquire.c)
   - Try class_hints[class_idx] FIRST before full metadata scan
   - Uses O(1) ss->shared_meta lookup for hint validation
   - __builtin_expect() for branch prediction optimization
   - 80-90% of acquire calls now skip full metadata scan

4. Proper Initialization (ss_allocation_box.c)
   - Initialize shared_meta = NULL in superslab_allocate()
   - Ensures correct NULL-check semantics for new SuperSlabs

Additional Improvements:
- Updated ptr_trace and debug ring for release build efficiency
- Enhanced ENV variable documentation and analysis
- Added learner_env_box.h for configuration management
- Various Box optimizations for reduced overhead

Thread Safety:
- All atomic operations use correct memory ordering
- shared_meta cached under mutex protection
- Lock-free Stage 2 uses proper CAS with acquire/release semantics

Testing:
- Benchmark: 1M iterations, 3.8M ops/s stable
- Build: Clean compile RELEASE=0 and RELEASE=1
- No crashes, memory leaks, or correctness issues

Next Optimization Candidates:
- P1: Per-SuperSlab free slot bitmap for O(1) slot claiming
- P2: Reduce Stage 2 critical section size
- P3: Page pre-faulting (MAP_POPULATE)

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

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

core/tiny_debug_ring.c

@@ -1,6 +1,8 @@
 #include "tiny_debug_ring.h"
 #include "hakmem_build_flags.h"
 #include "hakmem_tiny.h"
+#include "hakmem_trace_master.h"  // Phase 4c: Master trace control
+#include "hakmem_stats_master.h"  // Phase 4d: Master stats control
 #include <signal.h>
 #include <stdatomic.h>
 #include <unistd.h>
@@ -195,8 +197,8 @@ void tiny_debug_ring_init(void) {
     return;  // No env reads or hooks in release builds
 #endif
     if (g_tiny_ring_enabled) return;
-    const char* env = getenv("HAKMEM_TINY_TRACE_RING");
-    if (!(env && *env && env[0] != '0')) {
+    // Unified trace: HAKMEM_TINY_TRACE_RING or HAKMEM_TRACE=ring
+    if (!hak_trace_check("HAKMEM_TINY_TRACE_RING", "ring")) {
         return;
     }
     g_tiny_ring_enabled = 1;
@@ -234,8 +236,7 @@ static void tiny_debug_ring_dtor(void) {
 #if HAKMEM_BUILD_RELEASE
     return;  // Skip env access in release builds
 #endif
-    const char* e = getenv("HAKMEM_TINY_DUMP_RING_ATEXIT");
-    if (e && *e && e[0] != '0') {
+    if (hak_stats_check("HAKMEM_TINY_DUMP_RING_ATEXIT", "ring")) {
         tiny_debug_ring_dump(STDERR_FILENO, 0);
     }
 }