walkthrough: wire highlightLines into StopArtifact diff & whole-file views

[brendanwhit]

Summary

Wires highlightLines() (added in fed2816) into the walkthrough's StopArtifact views. The diff view and whole-file view already use it; the walkthrough's artifact diff and expanded whole-file view were missed in that commit, so multi-line tokens (docstrings, template literals, block comments) still render incorrectly inside walkthrough stops.

The wiring mirrors DiffView.precomputeHighlights exactly: each hunk is split into old-side (context + del) and new-side (context + add) blocks, each block is highlighted as one unit, and the resulting per-line HTML is passed to DiffLine via the existing highlightedHtml prop. The expanded whole-file view passes its full content as one block.

Repro

Open a walkthrough on any Python file with a multi-line `"""docstring"""`. Before this PR, words like dict, list, from inside the docstring body get keyword/type colors. After, the whole docstring body renders as a single string color.

Test plan

• Existing frontend tests pass (`npm run test:frontend`)
• `npm run lint` clean
• `npm run format:check` clean
• `npm run build` clean
• Manual: open a Python file with a multi-line docstring in walkthrough mode and confirm rendering

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