Module 02: cover the underscore-prefix 'private/internal' convention

从 02_function_classes.qmd 的命名约定部分附近开始，检查现有的 mikeio.pfs._pfssection.PfsSection 示例。添加关于下划线前缀、all、重新导出和双下划线的指导，然后更新 07_packaging.qmd 中的 Breaking changes 幻灯片，加入关于公共 API 的限定说明和交叉引用。完成的标准是两个模块都清楚地将内部名称约定与版本管理联系起来。

Module 02 (02_function_classes.qmd) teaches Python naming conventions but does not cover the leading-underscore convention for internal/private names. This came up in practice: a user upgraded a dependency, found that several _-prefixed functions had been removed, and was upset — not realising those were never part of the public API.

The breaking-changes slide in 07_packaging.qmd (Removing a function / Renaming / Changing signature → bump major) is the counterpart to this: removing a _private function is not a breaking change. Worth a forward reference between the two modules.

Suggested content for module 02

Add a slide near the existing naming-conventions section (around 02_function_classes.qmd:802) covering:

• _foo signals internal — not part of the public API
• Consumers who import underscore-prefixed names do so at their own risk
• Maintainers may change/remove them without bumping the major version
• __all__ in __init__.py to declare the public surface
• Re-exporting internals into the package namespace (the existing mikeio.pfs._pfssection.PfsSection example at 02_function_classes.qmd:795 is a natural lead-in)
• Double-underscore (__name) name mangling is a separate thing — mention briefly to avoid confusion

Cross-reference

Update the "Breaking changes" slide in 07_packaging.qmd to note that the rules apply to the public API only, with a pointer back to module 02.

See PEP 8 — Naming Conventions / Public and internal interfaces.