feat(dag): add DAG schema constraint and YAML syntax

[johanpel]

Description

Add quent-dag, a schema constraint for entity-based directed acyclic graphs.

• Define DAG, vertex, edge, membership, source, and target roles.
• Validate membership and endpoint types, cardinality, placement, and DAG ownership.
• Provide builders for DAG, vertex, and edge entities.
• Add YAML syntax for ordinary and FSM entities, including inferred DAG roots and generated topology references.
• Keep DAG root events user-defined.

The constraint validates schema-level topology. Instance-level acyclicity and reference integrity remain event-reconstruction concerns.

Related Issues

Part of #191

Testing

• cargo test -p quent-dag -p quent-yaml
• cargo clippy -p quent-dag -p quent-yaml --all-targets -- -D warnings
• cargo fmt --all --check
• RUSTDOCFLAGS='-D warnings' cargo doc -p quent-dag -p quent-yaml --no-deps

Written by Codex.