Split `flow.py` into DSL, definition, and runtime (#5997)

This commit separates the monolithic `flow.py` into three modules, each
with one job:

- `dsl.py` - the Python DSL for flows (@start/@listen/@router, or_/and_)
- `flow_definition.py` - the structural model extracted from the DSL
- `runtime.py` - the execution engine and state for flows

This phase moves code only and should not have any breaking changes.