Adopt file-backed dynamic package version

[adohe]

Background

PR #37 keeps the v0.0.2 release simple by leaving pyproject.toml with a static project.version and documenting the tag-driven publish workflow. That is good enough for the current release, but the next milestone should reduce version metadata duplication.

Peer projects such as verl and TRL use a file-backed dynamic package version: pyproject.toml declares dynamic = ["version"], and setuptools reads the version from a dedicated version file.

Scope

Move AReno to a file-backed dynamic version strategy after v0.0.2 is cut.

Expected shape:

• Add a single version source file, e.g. VERSION or areno/version/version.
• Update pyproject.toml to declare dynamic = ["version"].
• Configure [tool.setuptools.dynamic] version = { file = "..." }.
• Keep the publish workflow deriving the package version from the vX.Y.Z tag and updating the version file before building the sdist.
• Update RELEASE.md so the release checklist points to the version file as the committed source of truth.

Acceptance criteria

• pyproject.toml no longer hardcodes project.version.
• Local package metadata resolves to the version from the version file.
• The PyPI publish workflow still publishes a PEP 440 version derived from a leading-v git tag.
• RELEASE.md documents the new version source of truth.
• A CPU-safe test or metadata validation covers the dynamic version behavior.

Notes

• Do not include this in the v0.0.2 release-hygiene PR; this is intentionally a follow-up cleanup for the next milestone.
• This should follow the file-backed approach used by projects like verl and TRL, not the heavier tag-only setuptools-scm approach unless maintainers explicitly choose that later.

[YAO-001]

已完成，解决方案见 PR #471：

#471

实现方案：

• 新增根目录 VERSION，保留当前版本 0.0.6，作为已提交版本号的唯一来源。
• pyproject.toml 改为 dynamic = ["version"]，通过 [tool.setuptools.dynamic] 从 VERSION 读取版本，不再硬编码 project.version。
• PyPI 发布工作流继续从前导 v 的 tag 派生版本；构建前用 packaging.version.Version 验证并规范化为 PEP 440，再写入 VERSION 后构建 sdist。
• RELEASE.md 已将发布检查项和 source of truth 更新为 VERSION。
• 新增 CPU-safe 回归测试：在临时源码树中写入不同的合法版本，并调用 setuptools 元数据命令，确认解析结果确实来自 VERSION；同时让 VERSION 变更触发 CPU CI。

验证结果：

• 聚焦 CPU 测试：4 passed。
• 本地元数据解析：0.0.6。
• 隔离 sdist 构建成功；归档包含 areno-0.0.6/VERSION，PKG-INFO 包含 Version: 0.0.6。
• 两个修改后的 GitHub Actions workflow 均通过 YAML 解析，git diff --check 通过。

完整 pytest tests/ -k cpu 在本机 Windows 环境收集阶段被现有 POSIX-only termios 导入阻断，未运行任何测试；PR 已如实记录，Linux CI 会覆盖完整 CPU 套件。本次改动不涉及 GPU 路径。

提交：945d01d build: use file-backed package version