Review readability of Jujutsu changes

Review the Jujutsu changes for the target revision, and propose ways to improve the code — above all by cutting its line count, and further by making it more idiomatic and consistent with the patterns the surrounding code already uses.

Follow the common review instructions: @../review-common.md

Main goals

The leading goal is to reduce the code's line count. A small increase is fine when it buys a worthwhile readability, maintainability, or consistency gain; when the trade-off is roughly even, favor fewer lines.

Keeping that as the primary goal, also make the code:

• Lower in cognitive and cyclomatic complexity
• More idiomatic and least surprising for a reader used to the language and its conventions
• Consistent with the patterns the surrounding code already establishes — error handling, logging, naming, and structural conventions — preferring the established form even when a diverging one would work
• Easier to read and navigate
• Equivalent in intent and main effects; minor implementation side effects not covered by tests (logging statements, and other incidental behavioral details) may change

The work of updating tests is never an argument against a simplification: if code can be simplified but doing so requires changing tests, make the change anyway. Prefer less test coverage over high test coverage for complex code. However, when code is shaped a certain way only because there is no other way to test it, that is a valid reason to leave it as is.

Opportunity examples

• Delete a special case that earns nothing: a branch, parameter or abstraction handling a state the production code cannot reach, a generalization with a single caller, or configurability nothing varies
• Factor similar code together
• Rely on existing standard library code rather than custom code
• Use idiomatic, self-documenting interfaces; for example, Python __str__ or Rust Display rather than a custom to_string method
• Rely on types rather than unenforced conventions
• Align a diverging error-handling, logging, or naming pattern with the one prevailing in the module
• Move a symbol the change adds to the scope or module it belongs in: a shared one when it is general enough for other code to reuse, or the one it is most closely tied to, such as a method of the type it operates on, or a function of the module owning the data it handles
• Where it helps, split a large source file into smaller modules with clear responsibilities

Python specific

• Replace loops with list comprehensions or itertools functions
  • the more_itertools dependency can be added if it helps
• Where argparse supports it, express type constraints and cross-argument validation (for example mutually exclusive arguments) in the parser itself rather than in checks after parsing

Rust specific

• Use combinators liberally
  • the itertools dependency can be added if it helps

Additional instructions

Rank the items that remove the most lines first.

When proposing to delete code as unneeded, show what makes the state impossible: an invariant, a type, or the complete set of call sites. Without it, drop the item.

Severity

Anchor severity to how the code fails its purpose and its reader:

• critical — a revision that defeats its own purpose: its description sets out to simplify or shrink the code, yet it adds more production code than it removes, where removing code would reach that goal
• major — a shape that actively misleads: a name or a pattern that suggests behavior the code does not have, or density enough to hide a defect from a reader looking for one
• minor — everything else, including most line count savings