[codex] Fix Elixir 1.20 compile warnings

[mikehostetler]

What changed

• Renamed the dead-letter record() type to avoid overriding Elixir's built-in type.
• Removed unused logger requirements.
• Collapsed unreachable fallback clauses into internal case branches where defensive behavior is still useful.
• Reordered nil module normalization before the generic atom clause.

Why

Elixir 1.20 / OTP 29 surfaces these as compile warnings, and CI treats warnings as errors.

Validation

• MIX_ENV=test mix compile --warnings-as-errors
• Final full package sweep across the local monofolder passed under Elixir 1.20 / OTP 29.