Workflow IR and validation primitives for SimpleAgents.

This crate currently provides:

• A minimal canonical workflow IR (start, llm, tool, condition, loop, end)
• Deterministic normalization helpers
• Structural validation with actionable diagnostics
• Runtime execution for the minimal node set
• In-process worker protocol and bounded worker pool primitives
• Worker pool adapter hook for external (for example gRPC) worker clients

Example

use simple_agents_workflow::{Node, NodeKind, WorkflowDefinition, validate_and_normalize};

let workflow = WorkflowDefinition {
    version: "v0".into(),
    name: "hello".into(),
    nodes: vec![
        Node { id: "start".into(), kind: NodeKind::Start { next: "end".into() } },
        Node { id: "end".into(), kind: NodeKind::End },
    ],
};

let normalized = validate_and_normalize(&workflow).unwrap();
assert_eq!(normalized.name, "hello");