Fix C0/C7 class confusion: Upgrade C7 stride to 2048B and fix meta->class_idx initialization

Root Cause:
1. C7 stride was 1024B, unable to serve 1024B user requests (need 1025B with header)
2. New SuperSlabs start with meta->class_idx=0 (mmap zero-init)
3. superslab_init_slab() only sets class_idx if meta->class_idx==255
4. Multiple code paths used conditional assignment (if class_idx==255), leaving C7 slabs with class_idx=0
5. This caused C7 blocks to be misidentified as C0, leading to HDR_META_MISMATCH errors

Changes:
1. Upgrade C7 stride: 1024B → 2048B (can now serve 1024B requests)
2. Update blocks_per_slab[7]: 64 → 32 (2048B stride / 64KB slab)
3. Update size-to-class LUT: entries 513-2048 now map to C7
4. Fix superslab_init_slab() fail-safe: only reinitialize if class_idx==255 (not 0)
5. Add explicit class_idx assignment in 6 initialization paths:
   - tiny_superslab_alloc.inc.h: superslab_refill() after init
   - hakmem_tiny_superslab.c: backend_shared after init (main path)
   - ss_unified_backend_box.c: unconditional assignment
   - ss_legacy_backend_box.c: explicit assignment
   - superslab_expansion_box.c: explicit assignment
   - ss_allocation_box.c: fail-safe condition fix

Fix P0 refill bug:
- Update obsolete array access after Phase 3d-B TLS SLL unification
- g_tls_sll_head[cls] → g_tls_sll[cls].head
- g_tls_sll_count[cls] → g_tls_sll[cls].count

Results:
- HDR_META_MISMATCH: eliminated (0 errors in 100K iterations)
- 1024B allocations now routed to C7 (Tiny fast path)
- NXT_MISALIGN warnings remain (legacy 1024B SuperSlabs, separate issue)

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

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

core/box/ss_legacy_backend_box.c

@@ -125,6 +125,11 @@ void* hak_tiny_alloc_superslab_backend_legacy(int class_idx)
         for (int slab_idx = 0; slab_idx < cap; slab_idx++) {
             TinySlabMeta* meta = &chunk->slabs[slab_idx];
 
+            // Skip slabs that belong to a different class (or are uninitialized).
+            if (meta->class_idx != (uint8_t)class_idx) {
+                continue;
+            }
+
             if (meta->capacity == 0) {
                 continue;
             }
@@ -270,6 +275,10 @@ int expand_superslab_head(SuperSlabHead* head) {
 
     superslab_init_slab(new_chunk, 0, block_size, owner_tid);
 
+    // CRITICAL FIX: Explicitly set class_idx to avoid C0/C7 confusion.
+    // New SuperSlabs start with meta->class_idx=0 (mmap zero-init).
+    new_chunk->slabs[0].class_idx = (uint8_t)head->class_idx;
+
     // Initialize the next_chunk link to NULL
     new_chunk->next_chunk = NULL;