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_debug_ring.c

@@ -1,4 +1,5 @@
 #include "tiny_debug_ring.h"
+#include "hakmem_build_flags.h"
 #include "hakmem_tiny.h"
 #include <signal.h>
 #include <stdatomic.h>
@@ -7,6 +8,11 @@
 #include <sys/types.h>
 #include <ucontext.h>
 
+#if HAKMEM_BUILD_RELEASE && !HAKMEM_DEBUG_VERBOSE
+// In release builds without verbose debug, tiny_debug_ring.h provides
+// static inline no-op stubs. Avoid duplicate definitions here.
+#else
+
 #define TINY_RING_IGNORE(expr) do { ssize_t _tw_ret = (expr); (void)_tw_ret; } while(0)
 
 #define TINY_RING_CAP 4096u
@@ -213,3 +219,5 @@ static void tiny_debug_ring_dtor(void) {
         tiny_debug_ring_dump(STDERR_FILENO, 0);
     }
 }
+
+#endif // HAKMEM_BUILD_RELEASE && !HAKMEM_DEBUG_VERBOSE