Second iter of the heap-Str ABI milestone. Adds the UINT64_MAX sentinel-header short-circuit to ailang_rc_inc / ailang_rc_dec so the hs.1 packed-struct globals (whose first slot is i64 -1) flow safely through generic RC paths. Tasks: (1) author rc_str_field_in_adt_static.ail.json + confirm builds; (2) TDD cycle for the safety test + runtime guards; (3) full regression sweep.
