fix(cli): mock findFFmpeg in render tests for CI without ffmpeg (#1154)

* fix(cli): pre-flight FFmpeg check and propagate failed_stage on render errors

Add an early FFmpeg availability check in renderLocal() so users get a
clear error message before the render starts instead of a cryptic ENOENT
mid-render. Also thread job.failedStage through handleRenderError into
the render_error telemetry event so we can attribute failures to a
specific pipeline stage.

* fix(cli): consolidate FFmpeg pre-flight into renderLocal()

Remove the duplicate findFFmpeg() check from run() — renderLocal()
already validates FFmpeg availability before starting. Single source of
truth.

* fix(cli): mock findFFmpeg in render tests for CI runners without ffmpeg