Add Box 3 (Pointer Conversion Layer) and fix POOL_TLS_PHASE1 default

## Major Changes

### 1. Box 3: Pointer Conversion Module (NEW)
- File: core/box/ptr_conversion_box.h
- Purpose: Unified BASE ↔ USER pointer conversion (single source of truth)
- API: PTR_BASE_TO_USER(), PTR_USER_TO_BASE()
- Features: Zero-overhead inline, debug mode, NULL-safe, class 7 headerless support
- Design: Header-only, fully modular, no external dependencies

### 2. POOL_TLS_PHASE1 Default OFF (CRITICAL FIX)
- File: build.sh
- Change: POOL_TLS_PHASE1 now defaults to 0 (was hardcoded to 1)
- Impact: Eliminates pthread_mutex overhead on every free() (was causing 3.3x slowdown)
- Usage: Set POOL_TLS_PHASE1=1 env var to enable if needed

### 3. Pointer Conversion Fixes (PARTIAL)
- Files: core/box/front_gate_box.c, core/tiny_alloc_fast.inc.h, etc.
- Status: Partial implementation using Box 3 API
- Note: Work in progress, some conversions still need review

### 4. Performance Investigation Report (NEW)
- File: HOTPATH_PERFORMANCE_INVESTIGATION.md
- Findings:
  - Hotpath works (+24% vs baseline) after POOL_TLS fix
  - Still 9.2x slower than system malloc due to:
    * Heavy initialization (23.85% of cycles)
    * Syscall overhead (2,382 syscalls per 100K ops)
    * Workload mismatch (C7 1KB is 49.8%, but only C5 256B has hotpath)
    * 9.4x more instructions than system malloc

### 5. Known Issues
- SEGV at 20K-30K iterations (pre-existing bug, not related to pointer conversions)
- Root cause: Likely active counter corruption or TLS-SLL chain issues
- Status: Under investigation

## Performance Results (100K iterations, 256B)
- Baseline (Hotpath OFF): 7.22M ops/s
- Hotpath ON: 8.98M ops/s (+24% improvement ✓)
- System malloc: 82.2M ops/s (still 9.2x faster)

## Next Steps
- P0: Fix 20K-30K SEGV bug (GDB investigation needed)
- P1: Lazy initialization (+20-25% expected)
- P1: C7 (1KB) hotpath (+30-40% expected, biggest win)
- P2: Reduce syscalls (+15-20% expected)

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

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

core/hakmem_tiny_fastcache.inc.h

@@ -98,11 +98,14 @@ static inline __attribute__((always_inline)) void* tiny_fast_pop(int class_idx)
     } else {
         g_fast_count[class_idx] = 0;
     }
+    // CRITICAL FIX: Convert base -> user pointer for classes 0-6
     // Headerless class (1KB): clear embedded next pointer before returning to user
     if (__builtin_expect(class_idx == 7, 0)) {
         *(void**)head = NULL;
+        return head;  // C7: return base (headerless)
     }
-    return head;
+    // C0-C6: return user pointer (base+1)
+    return (void*)((uint8_t*)head + 1);
 }
 
 static inline __attribute__((always_inline)) int tiny_fast_push(int class_idx, void* ptr) {
@@ -144,7 +147,13 @@ static inline __attribute__((always_inline)) int tiny_fast_push(int class_idx, v
 static inline void* fastcache_pop(int class_idx) {
     TinyFastCache* fc = &g_fast_cache[class_idx];
     if (__builtin_expect(fc->top > 0, 1)) {
-        return fc->items[--fc->top];
+        void* base = fc->items[--fc->top];
+        // CRITICAL FIX: Convert base -> user pointer for classes 0-6
+        // FastCache stores base pointers, user needs base+1
+        if (class_idx == 7) {
+            return base;  // C7: headerless, return base
+        }
+        return (void*)((uint8_t*)base + 1);  // C0-C6: return user pointer
     }
     return NULL;
 }