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/hakmem_tiny_lifecycle.inc

@@ -144,25 +144,24 @@ void hak_tiny_trim(void) {
 static void tiny_tls_cache_drain(int class_idx) {
     TinyTLSList* tls = &g_tls_lists[class_idx];
 
-    // Drain TLS SLL cache (skip C7)
-    void* sll = (class_idx == 7) ? NULL : g_tls_sll_head[class_idx];
+    // Phase E1-CORRECT: Drain TLS SLL cache for ALL classes
+    #include "box/tiny_next_ptr_box.h"
+    void* sll = g_tls_sll_head[class_idx];
     g_tls_sll_head[class_idx] = NULL;
     g_tls_sll_count[class_idx] = 0;
     while (sll) {
-        #include "tiny_nextptr.h"
-        void* next = tiny_next_load(sll, class_idx);
+        void* next = tiny_next_read(class_idx, sll);
         tiny_tls_list_guard_push(class_idx, tls, sll);
         tls_list_push(tls, sll, class_idx);
         sll = next;
     }
 
-    // Drain fast tier cache (skip C7)
-    void* fast = (class_idx == 7) ? NULL : g_fast_head[class_idx];
+    // Phase E1-CORRECT: Drain fast tier cache for ALL classes
+    void* fast = g_fast_head[class_idx];
     g_fast_head[class_idx] = NULL;
     g_fast_count[class_idx] = 0;
     while (fast) {
-        #include "tiny_nextptr.h"
-        void* next = tiny_next_load(fast, class_idx);
+        void* next = tiny_next_read(class_idx, fast);
         tiny_tls_list_guard_push(class_idx, tls, fast);
         tls_list_push(tls, fast, class_idx);
         fast = next;
@@ -176,8 +175,7 @@ static void tiny_tls_cache_drain(int class_idx) {
         if (taken == 0u || head == NULL) break;
         void* cur = head;
         while (cur) {
-            #include "tiny_nextptr.h"
-            void* next = tiny_next_load(cur, class_idx);
+            void* next = tiny_next_read(class_idx, cur);
             SuperSlab* ss = hak_super_lookup(cur);
             if (ss && ss->magic == SUPERSLAB_MAGIC) {
                 hak_tiny_free_superslab(cur, ss);