Function build_start_config_json 

fn build_start_config_json(
    config_file: Option<&Path>,
    sets: &[String],
) -> Result<Option<String>>

Materialize a per-instance config from --config <file> plus repeatable --set k.v=val overrides. Returns the merged JSON string, or None when neither input was provided.

Layering: load file (json or yaml) → overlay each --set on the tree → serialise to a single JSON string. The string is delivered to the provider exclusively via Driver(CMD_INIT, config_json). The provider process MUST NOT read this through env / disk — that’s the v0.1 invariant rbnx start and rbnx boot both honour.