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.inc.h

@@ -21,6 +21,7 @@
 #include "tiny_region_id.h"            // Phase 7: Header-based class_idx lookup
 #include "tiny_adaptive_sizing.h"      // Phase 2b: Adaptive sizing
 #include "box/tls_sll_box.h"           // Box TLS-SLL: C7-safe push/pop/splice
+#include "box/tiny_next_ptr_box.h"     // Box API: Next pointer read/write
 #ifdef HAKMEM_TINY_FRONT_GATE_BOX
 #include "box/front_gate_box.h"
 #endif
@@ -202,14 +203,7 @@ static inline void* tiny_alloc_fast_pop(int class_idx) {
     }
 #endif
 
-    // CRITICAL: C7 (1KB) is headerless - delegate to slow path completely
-    // Reason: Fast path uses SLL which stores next pointer in user data area
-    //         C7's headerless design is incompatible with fast path assumptions
-    // Solution: Force C7 to use slow path for both alloc and free
-    if (__builtin_expect(class_idx == 7, 0)) {
-        return NULL;  // Force slow path
-    }
-
+    // Phase E1-CORRECT: C7 now has headers, can use fast path
 #ifdef HAKMEM_TINY_FRONT_GATE_BOX
     void* out = NULL;
     if (front_gate_try_pop(class_idx, &out)) {
@@ -351,12 +345,7 @@ static inline int sfc_refill_from_sll(int class_idx, int target_count) {
         }
 
         // Push to SFC (Layer 0) — header-aware
-#if HAKMEM_TINY_HEADER_CLASSIDX
-        const size_t sfc_next_off = (class_idx == 7) ? 0 : 1;
-#else
-        const size_t sfc_next_off = 0;
-#endif
-        *(void**)((uint8_t*)ptr + sfc_next_off) = g_sfc_head[class_idx];
+        tiny_next_write(class_idx, ptr, g_sfc_head[class_idx]);
         g_sfc_head[class_idx] = ptr;
         g_sfc_count[class_idx]++;
 
@@ -384,12 +373,7 @@ static inline int sfc_refill_from_sll(int class_idx, int target_count) {
 // - Smaller count (8-16): better for diverse workloads, faster warmup
 // - Larger count (64-128): better for homogeneous workloads, fewer refills
 static inline int tiny_alloc_fast_refill(int class_idx) {
-    // CRITICAL: C7 (1KB) is headerless - skip refill completely, force slow path
-    // Reason: Refill pushes blocks to TLS SLL which stores next pointer in user data
-    //         C7's headerless design is incompatible with this mechanism
-    if (__builtin_expect(class_idx == 7, 0)) {
-        return 0;  // Skip refill, force slow path allocation
-    }
+    // Phase E1-CORRECT: C7 now has headers, can use refill
 
     // Phase 7 Task 3: Profiling overhead removed in release builds
     // In release mode, compiler can completely eliminate profiling code