flow-step-development

添加具有成功/失败/跳过分支的流程步骤，覆盖配置、执行与注册全生命周期。

添加能根据结果走不同分支（success/error/skip）的流程步骤，包含 flow-config、worker handler、builder viewer 和步骤注册的全生命周期。需要基于步骤结果路由到下一个节点时用，适合复杂流程编排。重点是按状态路由并有 beforeStep 钩子，保证每步返回的 status 决定下一步走向。

# Flow Step Development

## Architecture Overview

A **step** is the unit of work inside a flow node. Steps execute sequentially within a node. After each step, the worker checks the result status and may route to a different node.

```
Node
├── beforeStep (optional)
└── steps[]
    ├── Step A → returns { status: "success" }  → routes to connected node via state.id
    ├── Step B → returns { status: "error" }    → routes to connected node via state.id
    └── Step C → returns { status: "wait" }     → pauses execution, resumes later
```

## State Design

States live **on the step**, not on the node. Each state has:

| Field | Purpose |
|-------|---------|
| `id` | React Flow **Handle ID** — u