Function read_config_value_from 

pub fn read_config_value_from(
    config: &Config,
    key: &str,
) -> Result<String, SubXError>

Read a single dot-notation configuration value from a Config snapshot.

This is the shared key-lookup table used by both the strict and the tolerant config get paths. Returns the value as a string (numerics are stringified, missing optional values are returned as the empty string), or Err for an unknown key.

Public because the subx-cli config get command is its other caller: the lookup table lives here with the Config type, and the CLI crate reaches it across the crate boundary.