Partial rendering

[zanjonke]

Summary

Adds partial rendering — when re-running codeplain on a module that has been previously rendered, detect what has changed and offer to resume from the right checkpoint instead of re-rendering everything from scratch.

Given a module tree (e.g. root → middle → leaf), on startup the tool now:

• Walks the tree looking for: (a) the last FRID successfully rendered, (b) any module whose plain spec has changed since its last render, (c) any module whose required modules' code has changed since.
• Picks the earliest-affected module and presents a TUI with the available continuation choices (continue from next FRID, re-render from start of changed module, re-render everything, quit).
• Feeds the user's choice into ModuleRenderer, which renders the chosen starting point before the root module.

The PR can be split in 3 parts:

• Refactoring of plain_modules.py and module_renderer.py
• Implementation of partial rendering
• Addiiton of unit tests

TODO: this is waiting on the design from @kaja-s DONE

Here are some preliminary screenshots.

[kaja-s]

Here are the final designs for all 13 situations with adjusted content: https://www.figma.com/design/7o1Ql87jRiT7k1Ne0Inugl/design?node-id=11155-2090