zero all new memory from allocations

zero all new memory from allocations

Change my_alloc() to use calloc instead of malloc so all fresh
allocations return zeroed memory. Also zero the expanded portion
in expand_item_list() after realloc, since it knows both old and
new sizes. This gives more predictable behaviour in case of bugs
where uninitialised or stale memory is accidentally accessed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>