Function execute_pipeline 

pub fn execute_pipeline(
    definition: &PipelineDefinition,
    files: Vec<PipelineFile>,
    registry: &NodeRegistry,
    reporter: &PipelineReporter,
    process_ctx: &dyn ProcessContext,
    now_ms: impl Fn() -> u64 + Copy,
) -> Result<PipelineResult, BntoError>

Execute a complete pipeline: walk nodes, iterate files, chain outputs.

Main entry point for the engine. now_ms is injected for testability (no std::time::Instant in WASM — uses js_sys::Date::now() instead).