feat: Build minimal DAG for multi-target execution

[Copilot]

When rendering to multiple output formats, each target's transformation path is computed independently — leading to redundant intermediate steps (e.g. Markdown→HTML executed twice when targeting both PDF and DOCX). This adds a MultiTargetDag that merges paths into a single shared execution graph.

Changes

src/graph/multi_target.rs (new)

• MultiTargetDag — merged DAG struct backed by petgraph::DiGraph
• merge_edge — inserts an edge for a (from, to) pair; keeps the cheaper edge on collision, discarding the duplicate
• execution_order — returns edges via topological sort (petgraph::algo::toposort), guaranteeing each step runs only after its inputs are ready
• Inspection helpers: contains_edge, edge_count, node_count, all_edges

src/graph/mod.rs

• TransformGraph::build_multi_target_dag(from, targets) — runs find_path (Dijkstra) per target, feeds all edges through merge_edge, returns None if any target is unreachable
• Exports MultiTargetDag from the public graph module

Example

let mut graph = TransformGraph::new();
graph.add_transform(TransformEdge::new(Format::Markdown, Format::Html, 0.5, 1.0));
graph.add_transform(TransformEdge::new(Format::Html, Format::Pdf,  0.8, 0.85));
graph.add_transform(TransformEdge::new(Format::Html, Format::Docx, 0.6, 0.90));

let dag = graph
    .build_multi_target_dag(Format::Markdown, &[Format::Pdf, Format::Docx])
    .unwrap();

assert_eq!(dag.edge_count(), 3); // Markdown→Html shared; 3 edges, not 4

let order = dag.execution_order(); // [Markdown→Html, Html→Pdf, Html→Docx]

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