Default integer inference to native width

[miselin]

Summary

• default unconstrained integer literals to the target profile's signed native word width
• range-check contextual integer literals at compile time without widening typed operands to fit literal values
• promote compatible binary operands to a shared width for arithmetic and comparisons
• apply the target C integer width to direct integer literals passed through varargs
• require explicit casts for incompatible mixed-signedness operations

Why

Exact-width literal inference allowed values such as 0 and 1 to become i1. That could truncate inferred accumulators and make ordinary comparisons such as 1 < 2 behave incorrectly after lowering.

This keeps unconstrained literals ergonomic while preserving a systems-language boundary: i16_value + 70000 is a compile-time error rather than a value-driven implicit widening.

Validation

• opam exec -- dune build
• opam exec -- dune runtest
• end-to-end compiled regression returned the expected result for inferred accumulation and comparison
• git diff --check

The Nix CI gates were not run because nix is unavailable in the current environment. Repository-wide dune fmt is also blocked by pre-existing formatting drift; unrelated promoted formatting changes were reverted.