walkthrough: vim-style line cursor (j/k/Ctrl-d/Ctrl-u/gg/G/c)

[brendanwhit]

Summary

Adds keyboard-driven line navigation to walkthrough mode so a reviewer can scan a stop without their hands leaving the keyboard.

Key	Action
j / k	Move cursor down / up one focusable diff line
Ctrl-d / Ctrl-u	Half-viewport jumps
gg / G	First / last focusable line of stop
c	Open comment composer on focused line

Existing bindings (Enter, Shift+Enter, g<1-9>, d, W, ?) unchanged. The new keys are scoped to walkthrough mode only; diff-mode bindings (j/k for the sidebar, c for commit picker) are untouched because the walkthrough block in useKeyboardShortcuts.ts early-returns before reaching them.

Focus model

A walkthroughCursor signal tracks { artifactIdx, rowIdx } for the active stop. rowIdx indexes only diff rows (line.type !== 'hunk'); hunk markers, the stop title, and the narrative scroll past silently. The cursor resets on stop change so j after Enter starts at the top of the next stop.

The focused row gets a 3px left-edge gutter bar and an 8% accent-tinted background, composited over the existing add/del/context colors. A createEffect in Stop.tsx calls scrollIntoView({ block: 'nearest' }) when the cursor moves, so manual scroll between keypresses isn't yanked.

c works by setting a one-shot commentTrigger signal that DiffLine watches — same code path as a mouse click on the row.

Test plan

• 14 unit tests for the cursor helpers (`walkthrough-cursor.test.ts`)
• Full suite passes (`npm test` → 233 passed, 9 skipped)
• `npm run lint` / `format:check` / `build` clean
• Manual: exercise j/k/gg/G/Ctrl-d/Ctrl-u/c in walkthrough mode; confirm Enter still advances stops and resets cursor; mouse click still opens comments

🤖 Generated with Claude Code