feat(ui): add toggleable line numbers in diff gutterLine numbers

[rashpile]

Summary

Adds side-by-side old/new line number columns to the diff gutter, toggled at runtime with L key.

Closes #20

What's changed

• New L key toggles line number display on/off
• Two-column gutter shows old and new line numbers ( OOO NNN format)
• Column width auto-adjusts per file based on max line number
• Works across all render modes: normal, wrapped (w), and collapsed (v)
• # status bar icon indicates when line numbers are active
• Help overlay (?) documents the L keybinding

Implementation

• lineNumbers bool and lineNumWidth int fields on the Model
• lineNumGutter(dl) helper formats the gutter using muted ANSI (raw sequences, no lipgloss reset)
• Gutter integrated into renderDiffLine, renderWrappedDiffLine, renderCollapsedAddLine, renderDeletePlaceholder
• Width recomputed in handleFileLoaded and toggleLineNumbers
• Guard for narrow terminals prevents negative content width in horizontal scroll

Test plan

• Unit tests for toggle, width computation, gutter formatting
• Integration tests for normal, wrapped, and collapsed modes with line numbers on/off
• End-to-end toggle test verifying rendered output
• Full test suite passes

Credits

powered by

• claude code
• ralphex

[rashpile]

Line numbers with annotations icon:

[umputun]

the line numbers feature itself looks good, two-column old/new is the right approach (we had single-column before and removed it because it was confusing in diff mode).

the branch needs a rebase on master though - it's based on a commit before --all-files/--exclude was merged, so the diff shows those files as "removed." once rebased the diff should be clean.

couple things to fix:

1. linter: var old, new string in diffview.go:30 - new shadows the Go builtin, rename to smth like oldCol/newCol
2. linter: untagged switch in diffview.go:31 - use switch dl.ChangeType { case ... } instead of switch { case dl.ChangeType == ... }
3. lineNumGutter comment says "raw ANSI" but uses m.styles.LineNumber.Render() (lipgloss). works fine in practice since the gutter is concatenated before content, but the comment is misleading - pls fix

[Copilot]

Pull request overview

Adds an optional old/new side-by-side line-number gutter to the diff view, toggleable at runtime with the L key, and updates rendering across normal/wrapped/collapsed modes plus documentation and tests.

Changes:

• Introduce Model.lineNumbers + Model.lineNumWidth, compute width per file, and add L key handling + status bar indicator.
• Prepend a two-column old/new line-number gutter to diff rendering (normal, wrapped, collapsed, delete placeholders), including width adjustments for wrapping and horizontal scroll.
• Add unit/integration-style tests for toggling, width computation, gutter formatting, and rendered output; update help/README/plugin usage docs.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ui/model.go	Adds model state + key handling for toggling and width recomputation on file load.
ui/model_test.go	Adds tests covering toggle behavior, width computation, help/status indicators, and an end-to-end render check.
ui/diffview.go	Implements gutter formatting and integrates it into normal/wrapped diff rendering and scroll/wrap width math.
ui/diffview_test.go	New tests for gutter formatting and rendering with/without line numbers (incl. wrap).
ui/collapsed.go	Integrates the gutter into collapsed-mode rendering paths and adjusts wrap/scroll calculations.
ui/collapsed_test.go	Adds a collapsed-mode render test with line numbers enabled.
ui/annotate.go	Adjusts delete-placeholder wrap-height accounting to include the extra gutter width.
README.md	Documents the new L keybinding and feature in the key table/features list.
CLAUDE.md	Updates internal notes/docs to include the new line-number mode and keybinding.
.claude-plugin/skills/revdiff/references/usage.md	Updates plugin usage docs with the L toggle keybinding.

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[rashpile]

Rebased on latest master — diff is clean now.

Fixes:

• var old, new → renamed to oldCol/newCol to avoid shadowing Go builtin new
• untagged switch → changed to switch dl.ChangeType { case ... }
• misleading comment → lineNumGutter comment now says "lipgloss style" instead of "raw ANSI", with note explaining why it's safe (gutter is concatenated before content, so the reset doesn't break outer backgrounds)
• toggleLineNumbers focus gate → now checks m.focus == paneDiff before toggling, aligned with toggleWrapMode/toggleCollapsedMode
• nestif lint → extracted wrap block into renderWrappedCollapsedLine() to reduce nesting complexity introduced by the line numbers guard

[umputun]

lgtm, all review points addressed. thx