Add core PhaseGraph DAG container for OpenHTF phases.

[copybara-service]

Add core PhaseGraph DAG container for OpenHTF phases.

• Introduce PhaseGraph (a PhaseCollectionNode) whose phases form a DAG: a prerequisites map declares which phases must finish before another may start, and phases with no unmet prerequisites may run concurrently.
• Add PhaseDependency(prerequisites, dependents) and PhaseGraph.from_dependencies() to build a graph from explicit dependencies; PhaseGraph.parallel() builds the degenerate graph with no dependencies (all phases eligible to run concurrently).
• Require unique phase names, detect cycles via a deterministic topological sort, and validate prerequisites in the constructor. Only phases may be nodes for now; collection nodes raise PhaseWrapError.
• Keep the prerequisites map synchronized with phase names across with_args(), with_plugs(), load_code_info(), apply_to_all_phases(), and copy(); include prerequisites in _asdict().
• Export PhaseGraph and PhaseDependency at the top-level openhtf module.

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