Describe parallel_chunks as chunk planning

## What to build

Align every explanation of `parallel_chunks` with the accepted chunking/fan-out boundary. The setting refines a query plan into more, smaller chunks; it does not execute those chunks concurrently. Fan-out performs execution, and the concurrency setting separately limits how many chunks are in flight.

Audit package documentation, API docstrings, configuration guidance, examples, and architecture prose. Preserve the public name and all behavior. Rewrite only passages that attribute execution or concurrency to the planning dial; leave already-correct division language unchanged.

## Acceptance criteria

- [ ] Every explanation frames `parallel_chunks` as optional chunk-plan refinement or a target chunk count.
- [ ] No explanation says the setting itself performs fan-out or parallel execution.
- [ ] Where performance is discussed, the prose clearly distinguishes more chunks from the separate concurrency cap and fan-out executor.
- [ ] Already-correct chunking descriptions remain intact.
- [ ] The public symbol, signatures, defaults, quota behavior, and execution behavior are unchanged.
- [ ] README and user-guide examples retain their existing calls and rendered behavior.
- [ ] Repository searches find no remaining planning/fan-out conflation around this setting.
- [ ] The documentation build, offline suite, type checker, linter, architecture tests, and dependency contracts pass.

## Blocked by

None — can start immediately.

首先在整个 repository 中搜索 `parallel_chunks`，涵盖 package 文档、API docstring、配置指南、示例和架构说明。只更新那些将 chunk 规划与 fan-out 或并发混淆的解释，保留正确的划分表述以及所有公开行为。验收搜索结果干净，并且文档构建、离线套件、类型检查器、linter、架构测试和依赖契约全部通过后，即视为完成。

由索引模型根据 Issue 内容生成。