Document why Object/Null tera values are dropped in answers serialization

[raiderrobert]

Context

In src/answers/mod.rs (lines 149-166), tera_value_to_toml silently drops Object and Null values:

fn tera_value_to_toml(value: &Value) -> Option<toml::Value> {
    match value {
        ...
        _ => None, // Silently drops Object and Null
    }
}

This is likely intentional since TOML tables would complicate the flat [variables] section, but there's no documentation explaining the choice.

Suggestion

Add a doc comment or inline comment explaining the intentional omission, and optionally emit a debug log when values are dropped so template authors can understand why certain variables don't appear in the answers file.