Read an umbrella's declared inter-app dependency graph out of the sandbox.

Mutare.Project.app_test_scopes/3 narrows a broad (whole-umbrella) mutant run to the mutant's owning app plus its transitive dependents, which needs the forward graph %{app => [sibling apps it depends on]}. This module gets it from Mix: one mix eval in the sandbox (no compile, no deps check) merges Mix.Project.deps_tree/0 from the umbrella root with sibling applications named by each child's application/0. Every child's deps/0 and application/0 are evaluated under MIX_ENV=test exactly as the test runs see them. Consequently, a runtime: false sibling and one named only through :extra_applications or explicit :applications all count as dependencies, while an only: that excludes the test env does not. Why the graph is read this way rather than only off the build: NOTES "Umbrella narrowing must follow the declared graph".

Output contract

The evaluated snippet prints one mutare-dep <app> <dep> <dep>… line per node of the tree and a closing mutare-dep-end line. parse/2 keeps the lines naming an umbrella app, drops deps that aren't umbrella apps (Hex packages), matches names as strings against the known app list (never String.to_atom/1 on subprocess output), and is :error unless every umbrella app was reported and the end marker arrived — a missing node would silently lose that app's edges, so any incompleteness means "unknown graph" and the caller runs the whole umbrella.