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/box/ptr_conversion_box.h

@@ -30,9 +30,10 @@
 
 /**
  * Convert BASE pointer (storage) to USER pointer (returned to caller)
+ * Phase E1-CORRECT: ALL classes (0-7) have 1-byte headers
  *
  * @param base_ptr Pointer to block in storage (no offset)
- * @param class_idx Size class (0-6: +1 offset, 7: +0 offset)
+ * @param class_idx Size class (0-7: +1 offset for all)
  * @return USER pointer (usable memory address)
  */
 static inline void* ptr_base_to_user(void* base_ptr, uint8_t class_idx) {
@@ -40,14 +41,7 @@ static inline void* ptr_base_to_user(void* base_ptr, uint8_t class_idx) {
         return NULL;
     }
 
-    /* Class 7 (2KB) is headerless - no offset */
-    if (class_idx == 7) {
-        PTR_CONV_LOG("BASE→USER cls=%u base=%p → user=%p (headerless)\n",
-                     class_idx, base_ptr, base_ptr);
-        return base_ptr;
-    }
-
-    /* Classes 0-6 have 1-byte header - skip it */
+    /* Phase E1-CORRECT: All classes 0-7 have 1-byte header - skip it */
     void* user_ptr = (void*)((uint8_t*)base_ptr + 1);
     PTR_CONV_LOG("BASE→USER cls=%u base=%p → user=%p (+1 offset)\n",
                  class_idx, base_ptr, user_ptr);
@@ -56,9 +50,10 @@ static inline void* ptr_base_to_user(void* base_ptr, uint8_t class_idx) {
 
 /**
  * Convert USER pointer (from caller) to BASE pointer (storage)
+ * Phase E1-CORRECT: ALL classes (0-7) have 1-byte headers
  *
  * @param user_ptr Pointer from user (may have +1 offset)
- * @param class_idx Size class (0-6: -1 offset, 7: -0 offset)
+ * @param class_idx Size class (0-7: -1 offset for all)
  * @return BASE pointer (block start in storage)
  */
 static inline void* ptr_user_to_base(void* user_ptr, uint8_t class_idx) {
@@ -66,14 +61,7 @@ static inline void* ptr_user_to_base(void* user_ptr, uint8_t class_idx) {
         return NULL;
     }
 
-    /* Class 7 (2KB) is headerless - no offset */
-    if (class_idx == 7) {
-        PTR_CONV_LOG("USER→BASE cls=%u user=%p → base=%p (headerless)\n",
-                     class_idx, user_ptr, user_ptr);
-        return user_ptr;
-    }
-
-    /* Classes 0-6 have 1-byte header - rewind it */
+    /* Phase E1-CORRECT: All classes 0-7 have 1-byte header - rewind it */
     void* base_ptr = (void*)((uint8_t*)user_ptr - 1);
     PTR_CONV_LOG("USER→BASE cls=%u user=%p → base=%p (-1 offset)\n",
                  class_idx, user_ptr, base_ptr);