Verify clean build under Elixir 1.20

[ausimian]

Summary

Check that Emily builds cleanly under Elixir 1.20.

The project currently declares elixir: "~> 1.18" in mix.exs, so 1.20 already satisfies the version requirement. We should confirm there are no new warnings, deprecations, or compile errors introduced by the 1.20 compiler and standard library.

Checklist

• Install Elixir 1.20 (with a compatible OTP release) and run a full mix compile --warnings-as-errors
• Run mix precommit (compile w/ warnings-as-errors, deps.unlock --unused, format, credo --strict, docs check, test)
• Review output for any new deprecation warnings from Elixir 1.20
• Confirm mix docs --warnings-as-errors (the :docs env gate) still passes
• Add Elixir 1.20 to the CI build matrix if it builds cleanly
• Bump the elixir: requirement in mix.exs only if dropping older versions is intended (otherwise leave ~> 1.18)

Notes

• Native build (libmlx / NIF) is macOS / Apple Silicon only, so a full source build needs that environment; pure-Elixir compile and warning checks can run more broadly.
• No code changes are expected unless 1.20 surfaces deprecations.