feat(graph): Implement Dijkstra-based pathfinding engine for format transformations

[Copilot]

Adds graph pathfinding to TransformGraph so callers can compute optimal transformation pipelines between document formats, with cost (additive) and quality (multiplicative) scoring.

Changes

src/graph/pathfinding.rs — new

• TransformPath struct: steps: Vec<TransformEdge>, total_cost: f32 (sum), total_quality: f32 (product)

src/graph/mod.rs — extended

• find_path(from, to) -> Option<TransformPath> — Dijkstra (A* with zero heuristic via petgraph) returning the minimum-cost path
• find_all_paths(from, to) -> Vec<TransformPath> — enumerates all simple paths via all_simple_paths, sorted by total_cost ascending
• edges_from_node_path — private helper that reconstructs the cheapest directed edge per node-pair, shared between both public methods

Usage

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::Markdown, Format::Pdf,  5.0, 0.90));

// Optimal (cheapest) path: Markdown → Html → Pdf
let path = graph.find_path(Format::Markdown, Format::Pdf).unwrap();
assert_eq!(path.steps.len(), 2);
assert!((path.total_cost    - 1.3 ).abs() < 1e-5); // additive
assert!((path.total_quality - 0.85).abs() < 1e-5); // multiplicative

// All candidate paths, sorted cheapest first
let candidates = graph.find_all_paths(Format::Markdown, Format::Pdf);
assert_eq!(candidates.len(), 2);

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