Function render_dot 

pub fn render_dot(graph: &AuthorityGraph, filter_job: Option<&str>) -> String

Render the authority graph as a Graphviz DOT digraph string.

When filter_job is Some(name), restricts the output to the subgraph reachable (in either edge direction) from any Step node whose META_JOB_NAME metadata equals name. When None, includes every node and edge.

Output is deterministic — nodes and edges are emitted in their stored (insertion) order, which makes the result diff-friendly and testable.