diff: add grouped diff gutter actions

[hewigovens]

Summary

• Add a GitHub Desktop-style group gutter beside the old/new line gutters in native diffs.
• Support selecting and abandoning contiguous change groups from the group gutter and context menu.
• Draw connected, mild blue group stripes through the gutter layout manager and cover grouping/rendering behavior with tests.

Validation

• swift test in shell/mac/Packages/JayJayDiffUI
• just build

[gemini-code-assist]

Code Review

This pull request introduces a new 'group' column in the diff gutter, featuring a vertical stripe to visually represent contiguous change hunks. The implementation includes a new DiffGutterGrouping utility for range expansion, updates to DiffGutterTextView for handling interactions in the new gutter regions, and enhancements to DiffLayoutManager for stripe rendering. Additionally, gutter-related actions and types were refactored into dedicated files. Review feedback suggests avoiding hardcoded colors in the view logic to support better theming and optimizing the measurement of gutter line widths by using the attributed string's size directly.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: a02493bc00

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".