Two builtins ship together: int_to_str (new) and float_to_str (today type-installed but codegen-deferred). Both produce malloc-backed, refcounted Str slabs via new ailang_int_to_str / ailang_float_to_str runtime exports.
Slab layout is {rc_header, len, bytes, NUL}. Static-Str globals migrate to the same packed-struct shape with a UINT64_MAX sentinel rc_header; ailang_rc_inc / _dec short-circuit on the sentinel. From every caller's POV static and heap Str values are indistinguishable, so existing code paths (eq__Str, compare__Str, io/print_str, per-type drop walks) keep working with a single +8 GEP at the three C-API callsites and no change to drop dispatch.
Out of scope: ++ concat operator, Show typeclass + print rewire, length-aware comparison for embedded-NUL Strs.
Brummel
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