Initial commit: justpipe - lightweight async orchestration for AI

This release marks the stable foundation of justpipe (v0.5.0), a lightweight, 
type-safe asynchronous orchestration framework designed for AI workflows and 
complex logic pipelines.

Key Features:
- Declarative API: Define pipelines using @pipe.map, @pipe.switch, and @pipe.on_error.
- Visualization: Integrated Mermaid.js support via an intermediate Visual AST.
- Reliability: Built-in support for retries, backpressure, and graceful shutdowns.
- Observability: Middleware system, lifecycle hooks, and pipeline introspection.
- Subpipelines: First-class support for nesting and modular pipeline design.
- Async Native: Fully leverages Python's asyncio for high-performance execution.
