Introduce Node, Connection, and Graph classes for defining pipelines
as graphs instead of verbose XYZStage naming convention.
- Add NodeType enum (SOURCE, CAMERA, EFFECT, DISPLAY, etc.)
- Add Node, Connection, and Graph dataclasses with type hints
- Add validation for cycles and disconnected nodes using DFS
- Add factory methods: node(), connect(), chain() for easy graph building
- Support for both imperative and declarative graph construction
This provides the foundation for the graph-based DSL that replaces
the verbose XYZStage naming convention with intuitive node-and-connection syntax.
David Gwilliam
c790027ede