[diffs/edit]: keep virtual EOF caret anchored after line reinsertion

[ije]

Reproduction steps:
0, Open the demo page, enable editing, and jump to the last line.
1, Press Enter to insert a new line.
2, Press Delete to remove the newly added final line.
3, [Bug] Press Enter again. The page scrolls to the top, and the caret does not remain focused on the newly inserted line.

this PR fix that issue by:

• Reset cached line geometry after every line-count change.
• Prevent a removed EOF row from being reused to position the caret at the top.
• Add regression coverage for Enter → Delete → Enter in a 1,000-line virtualized file.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
pierre-docs-diffs	Ready	Preview	Jul 21, 2026 12:49pm
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pierrejs-diff-demo	Ready	Preview	Jul 21, 2026 12:49pm

[ije]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

Reviewed commit: 072eb4ab5a

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