First iter of the heap-Str ABI milestone (docs/specs/2026-05-12-heap-str-abi.md). Carrier reversed by Boss judgement: original carrier said runtime+checker first, but the spec's no-intermediate-state clause forbids installing int_to_str without its codegen arm, and the runtime/str.c additions can't be tested in isolation. Layout migration is the natural prerequisite that everything else builds on.
Tasks: (1) intern_str_literal table + function + emission loop + Literal::Str callsite switch; (2-4) +8 GEPs at io/print_str, eq__Str, compare__Str callsites; (5) snapshot regen + cross_lang.py regression check. Acceptance: every existing program byte-identical.
Brummel
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