Error out if TOML includes any unknown fields

Currently, the toml parser just silently ignores any fields that don't match the struct. This means that typos are hard to diagnose, and backwards-incompatible changes to the config will require special care to notify existing users.

Locate the TOML parser entry point and the configuration structs it populates. Add coverage for unknown fields, including the expected error, and confirm that valid configuration fields continue to parse successfully.

Written by the indexing model from the issue text.