Spec a Rust config loader with useful errors

Your Rust CLI tool reads its settings from config.yaml. Users keep filing bugs like "my timeout setting does nothing", and a broken file makes the tool panic with an unreadable message. An agent will rewrite the loader: parse the file, check it, apply defaults and report errors.

Write the spec you would hand the agent.

What you know:

• Rust 2021, with serde and serde_yaml already dependencies
• server.host is required, and server.port defaults to 8080
• timeout_secs defaults to 30 and must be 1 to 300, and retries defaults to 3
• region is a short lowercase code such as us, de or no

Then, in a few sentences: what does a vague "parse the YAML config" prompt get wrong here?