Function run_headless 

pub fn run_headless(
    entry: Box<dyn Fn()>,
    modules: Vec<Box<dyn RenderModule>>,
    config: TesseraConfig,
) -> Result<(), HeadlessError>

Runs the worker side of the cargo tessera headless protocol.

Reads one JSON command per line from stdin and writes one JSON response per line to stdout. Diagnostics are written to stderr so stdout stays a clean JSONL stream.