Phase E3-FINAL: Fix Box API offset bugs - ALL classes now use correct offsets

## Root Cause Analysis (GPT5)

**Physical Layout Constraints**:
- Class 0: 8B = [1B header][7B payload] → offset 1 = 9B needed = ❌ IMPOSSIBLE
- Class 1-6: >=16B = [1B header][15B+ payload] → offset 1 = ✅ POSSIBLE
- Class 7: 1KB → offset 0 (compatibility)

**Correct Specification**:
- HAKMEM_TINY_HEADER_CLASSIDX != 0:
  - Class 0, 7: next at offset 0 (overwrites header when on freelist)
  - Class 1-6: next at offset 1 (after header)
- HAKMEM_TINY_HEADER_CLASSIDX == 0:
  - All classes: next at offset 0

**Previous Bug**:
- Attempted "ALL classes offset 1" unification
- Class 0 with offset 1 caused immediate SEGV (9B > 8B block size)
- Mixed 2-arg/3-arg API caused confusion

## Fixes Applied

### 1. Restored 3-Argument Box API (core/box/tiny_next_ptr_box.h)
```c
// Correct signatures
void tiny_next_write(int class_idx, void* base, void* next_value)
void* tiny_next_read(int class_idx, const void* base)

// Correct offset calculation
size_t offset = (class_idx == 0 || class_idx == 7) ? 0 : 1;
```

### 2. Updated 123+ Call Sites Across 34 Files
- hakmem_tiny_hot_pop_v4.inc.h (4 locations)
- hakmem_tiny_fastcache.inc.h (3 locations)
- hakmem_tiny_tls_list.h (12 locations)
- superslab_inline.h (5 locations)
- tiny_fastcache.h (3 locations)
- ptr_trace.h (macro definitions)
- tls_sll_box.h (2 locations)
- + 27 additional files

Pattern: `tiny_next_read(base)` → `tiny_next_read(class_idx, base)`
Pattern: `tiny_next_write(base, next)` → `tiny_next_write(class_idx, base, next)`

### 3. Added Sentinel Detection Guards
- tiny_fast_push(): Block nodes with sentinel in ptr or ptr->next
- tls_list_push(): Block nodes with sentinel in ptr or ptr->next
- Defense-in-depth against remote free sentinel leakage

## Verification (GPT5 Report)

**Test Command**: `./out/release/bench_random_mixed_hakmem --iterations=70000`

**Results**:
- ✅ Main loop completed successfully
- ✅ Drain phase completed successfully
- ✅ NO SEGV (previous crash at iteration 66151 is FIXED)
- ℹ️ Final log: "tiny_alloc(1024) failed" is normal fallback to Mid/ACE layers

**Analysis**:
- Class 0 immediate SEGV: ✅ RESOLVED (correct offset 0 now used)
- 66K iteration crash: ✅ RESOLVED (offset consistency fixed)
- Box API conflicts: ✅ RESOLVED (unified 3-arg API)

## Technical Details

### Offset Logic Justification
```
Class 0:  8B block → next pointer (8B) fits ONLY at offset 0
Class 1: 16B block → next pointer (8B) fits at offset 1 (after 1B header)
Class 2: 32B block → next pointer (8B) fits at offset 1
...
Class 6: 512B block → next pointer (8B) fits at offset 1
Class 7: 1024B block → offset 0 for legacy compatibility
```

### Files Modified (Summary)
- Core API: `box/tiny_next_ptr_box.h`
- Hot paths: `hakmem_tiny_hot_pop*.inc.h`, `tiny_fastcache.h`
- TLS layers: `hakmem_tiny_tls_list.h`, `hakmem_tiny_tls_ops.h`
- SuperSlab: `superslab_inline.h`, `tiny_superslab_*.inc.h`
- Refill: `hakmem_tiny_refill.inc.h`, `tiny_refill_opt.h`
- Free paths: `tiny_free_magazine.inc.h`, `tiny_superslab_free.inc.h`
- Documentation: Multiple Phase E3 reports

## Remaining Work

None for Box API offset bugs - all structural issues resolved.

Future enhancements (non-critical):
- Periodic `grep -R '*(void**)' core/` to detect direct pointer access violations
- Enforce Box API usage via static analysis
- Document offset rationale in architecture docs

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

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

core/tiny_alloc_fast_inline.h

@@ -10,7 +10,7 @@
 #include <stdint.h>
 #include "hakmem_build_flags.h"
 #include "tiny_remote.h"  // for TINY_REMOTE_SENTINEL (defense-in-depth)
-#include "tiny_nextptr.h"
+#include "box/tiny_next_ptr_box.h"  // Phase E1-CORRECT: unified next pointer API
 #include "tiny_region_id.h"  // For HEADER_MAGIC, HEADER_CLASS_MASK (Fix #7)
 
 // External TLS variables (defined in hakmem_tiny.c)
@@ -52,16 +52,14 @@ extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
             if (g_tls_sll_count[(class_idx)] > 0) g_tls_sll_count[(class_idx)]--; \
             (ptr_out) = NULL; \
         } else { \
-            /* Safe load of header-aware next (avoid UB on unaligned) */ \
-            void* _next = tiny_next_load(_head, (class_idx)); \
+            /* Phase E1-CORRECT: Use Box API for next pointer read */ \
+            void* _next = tiny_next_read(class_idx, _head); \
             g_tls_sll_head[(class_idx)] = _next; \
             if (g_tls_sll_count[(class_idx)] > 0) { \
                 g_tls_sll_count[(class_idx)]--; \
             } \
-            (ptr_out) = _head; \
-            if (__builtin_expect((class_idx) == 7, 0)) { \
-                *(void**)(ptr_out) = NULL; \
-            } \
+            /* Phase E1-CORRECT: All classes return user pointer (base+1) */ \
+            (ptr_out) = (void*)((uint8_t*)_head + 1); \
         } \
     } else { \
         (ptr_out) = NULL; \
@@ -85,21 +83,19 @@ extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];
 //     mov    %rsi, g_tls_sll_head(%rdi)
 //
 #if HAKMEM_TINY_HEADER_CLASSIDX
-// ✅ FIX #7: Restore header on FREE (header-mode enabled)
+// Phase E1-CORRECT: Restore header on FREE for ALL classes (including C7)
 // ROOT CAUSE: User may have overwritten byte 0 (header). tls_sll_splice() checks
 // byte 0 for HEADER_MAGIC. Without restoration, it finds 0x00 → uses wrong offset → SEGV.
 // COST: 1 byte write (~1-2 cycles per free, negligible).
 #define TINY_ALLOC_FAST_PUSH_INLINE(class_idx, ptr) do { \
-    if ((class_idx) != 7) { \
-        *(uint8_t*)(ptr) = HEADER_MAGIC | ((class_idx) & HEADER_CLASS_MASK); \
-    } \
-    tiny_next_store((ptr), (class_idx), g_tls_sll_head[(class_idx)]); \
+    *(uint8_t*)(ptr) = HEADER_MAGIC | ((class_idx) & HEADER_CLASS_MASK); \
+    tiny_next_write(class_idx, (ptr), g_tls_sll_head[(class_idx)]); \
     g_tls_sll_head[(class_idx)] = (ptr); \
     g_tls_sll_count[(class_idx)]++; \
 } while(0)
 #else
 #define TINY_ALLOC_FAST_PUSH_INLINE(class_idx, ptr) do { \
-    tiny_next_store((ptr), (class_idx), g_tls_sll_head[(class_idx)]); \
+    tiny_next_write(class_idx, (ptr), g_tls_sll_head[(class_idx)]); \
     g_tls_sll_head[(class_idx)] = (ptr); \
     g_tls_sll_count[(class_idx)]++; \
 } while(0)