@spec build([Oi.Compiler.RecipeBundle.t()]) :: {:ok, Oi.Workspace.Planning.Plan.t()}

Build a Plan from a flat list of RecipeBundles.

Bundles are grouped by their position in the compiled output. Within a single workspace, all bundles form a linear sequence (one bundle per cluster). Bundles at the same index across different compilations would run in the same stage.

For a single workspace (one graph → one compile call), each bundle is its own stage — they execute sequentially in topo order.