Layered configuration loader.

Minimal env-var loader for the Gateway. Adds layered file (yaml/toml) and CLI-flag merging in later iterations when those callers exist.

Reading

use tt_config::Config;
let cfg = Config::from_env().expect("env vars valid");

Conventions

• Required vars return [ConfigError::Missing] when absent.
• Optional vars return None when absent and Some(_) when present.
• Parse failures (e.g. PORT="abc") return [ConfigError::Parse].