Refactor: Phase 2 Box化 - SuperSlab Lookup Box with multiple contract levels

Purpose: Formalize SuperSlab lookup responsibilities with clear safety guarantees

Evolution:
- Phase 12: UNSAFE mask+dereference (5-10 cycles) → 12% crash rate
- Phase 1b: SAFE registry lookup (50-100 cycles) → 0% crash rate
- Phase 2: Box化 - multiple contracts (UNSAFE/SAFE/GUARDED)

Box Pattern Benefits:
1. Clear Contracts: Each API documents preconditions and guarantees
2. Multiple Levels: Choose speed vs safety based on context
3. Future-Proof: Enables optimizations without breaking existing code

API Design:
- ss_lookup_unsafe(): 5-10 cycles, requires validated pointer (internal use only)
- ss_lookup_safe(): 50-100 cycles, works with arbitrary pointers (recommended)
- ss_lookup_guarded(): 100-200 cycles, adds integrity checks (debug only)
- ss_fast_lookup(): Backward compatible (→ ss_lookup_safe)

Implementation:
- Created core/box/superslab_lookup_box.h with full contract documentation
- Integrated into core/superslab/superslab_inline.h
- ss_lookup_safe() implemented as macro to avoid circular dependency
- ss_lookup_guarded() only available in debug builds
- Removed conflicting extern declarations from 3 locations

Testing:
- Build: Success (all warnings resolved)
- Crash rate: 0% (50/50 iterations passed)
- Backward compatibility: Maintained via ss_fast_lookup() macro

Future Optimization Opportunities (documented in Box):
- Phase 2.1: Hybrid lookup (try UNSAFE first, fallback to SAFE)
- Phase 2.2: Per-thread cache (1-2 cycles hit rate)
- Phase 2.3: Hardware-assisted validation (PAC/CPUID)

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

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

core/box/external_guard_box.h

@@ -104,7 +104,7 @@ static inline int external_guard_try_free(void* ptr) {
         }
 
         // Debug: Check if this looks like a HAKMEM allocation
-        extern SuperSlab* hak_super_lookup(void*);
+        // Note: hak_super_lookup() is defined in hakmem_super_registry.h (included transitively)
         SuperSlab* ss = hak_super_lookup(ptr);
         fprintf(stderr, "[ExternalGuard] hak_super_lookup(ptr) = %p\n", (void*)ss);
         if (ss) {