Preprocesses <<: *anchor merge keys before serde_yaml 0.9 deserialization.
serde_yaml implements YAML 1.2 which dropped merge keys; the yaml-merge-keys
crate resolves them as a preprocessing step, giving full anchor + merge
support for DRY pipeline definitions.
YamlWorkflowFile supports both single (workflow:) and multi (workflows:)
formats. WorkflowSpec gains typed inputs/outputs declarations.
Type string parser for inline types ("string?", "list<number>", etc.).
load_workflow_from_str returns Vec<CompiledWorkflow>.
Backward-compatible load_single_workflow_from_str convenience function.