Fix TLS-SLL splice alignment issue causing SIGSEGV

- core/box/tls_sll_box.h: Normalize splice head, remove heuristics, fix misalignment guard
- core/tiny_refill_opt.h: Add LINEAR_LINK debug logging after carve
- core/ptr_trace.h: Fix function declaration conflicts for debug builds
- core/hakmem.c: Add stdatomic.h include and ptr_trace_dump_now declaration

Fixes misaligned memory access in splice_trav that was causing SIGSEGV.
TLS-SLL GUARD identified: base=0x7244b7e10009 (should be 0x7244b7e10401)
Preserves existing ptr=0xa0 guard for small pointer free detection.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

core/tiny_refill_opt.h

@@ -230,6 +230,17 @@ static inline uint32_t trc_linear_carve(uint8_t* base, size_t bs,
         cursor = next;
     }
     void* tail = (void*)cursor;
+    // Debug: validate first link
+#if !HAKMEM_BUILD_RELEASE
+    if (batch >= 2) {
+        void* first_next = *(void**)((uint8_t*)head + next_offset);
+        fprintf(stderr, "[LINEAR_LINK] cls=%d head=%p off=%zu next=%p tail=%p\n",
+                class_idx, head, (size_t)next_offset, first_next, tail);
+    } else {
+        fprintf(stderr, "[LINEAR_LINK] cls=%d head=%p off=%zu next=%p tail=%p\n",
+                class_idx, head, (size_t)next_offset, (void*)0, tail);
+    }
+#endif
     // FIX: Update both carved (monotonic) and used (active count)
     meta->carved += batch;
     meta->used += batch;