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/tiny_free_fast.inc.h

@@ -40,6 +40,9 @@ extern pthread_t tiny_self_pt(void);
 // External TLS variables (from Box 5)
 extern __thread void* g_tls_sll_head[TINY_NUM_CLASSES];
 extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
+// Hot-class toggle: class5 (256B) dedicated TLS fast path
+extern int g_tiny_hotpath_class5;
+extern __thread TinyTLSList g_tls_lists[TINY_NUM_CLASSES];
 
 // Box 5 helper (TLS push)
 extern void tiny_alloc_fast_push(int class_idx, void* ptr);
@@ -124,10 +127,13 @@ static inline int tiny_free_fast_ss(SuperSlab* ss, int slab_idx, void* ptr, uint
     g_free_via_ss_local[class_idx]++;
 #endif
 
-    // Box 5-NEW/5-OLD integration: Push to TLS freelist (SFC or SLL)
+    // Box 5 integration: class5 can use dedicated TLS List hotpath
     extern int g_sfc_enabled;
-    if (g_sfc_enabled) {
-        // Box 5-NEW: Try SFC (128 slots)
+    if (__builtin_expect(g_tiny_hotpath_class5 && class_idx == 5, 0)) {
+        TinyTLSList* tls5 = &g_tls_lists[5];
+        tls_list_push_fast(tls5, base, 5);
+    } else if (g_sfc_enabled) {
+        // Box 5-NEW: Try SFC (128-256 slots)
         if (!sfc_free_push(class_idx, base)) {
             // SFC full → skip caching, use slow path (return 0)
             // Do NOT fall back to SLL - it has no capacity check and would grow unbounded!