Preserve transcript layout caches during backtrack selection

[copyberry]

Preserve transcript layout caches during backtrack selection

Why

Moving between prompts in backtrack mode rebuilt every transcript renderable and
discarded their cached heights, causing the entire transcript to be laid out
again for each selection change.

What changed

Rerender only the previously highlighted cell and the newly highlighted cell.
Keep all other renderables, cached heights, and the live tail intact while
continuing to scroll the selected prompt into view.

Testing

Add regression tests that verify selection changes preserve unrelated height
caches and produce the same viewport and scroll position as a full rebuild,
including with a live tail and terminal width changes.

[github-actions]

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