refactor(view): panel cursor save/restore helpers

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR refactors duplicated “panel cursor save/restore” logic into StandardView helper methods, and updates view listeners to use the shared implementation. It also adds functional tests to validate the new helpers’ behavior.

Changes:

• Add StandardView:save_panel_cursor() and StandardView:restore_panel_cursor() helper methods.
• Replace duplicated cursor persistence logic in diff + file history listeners with calls to the new helpers.
• Add functional tests covering save/restore behavior and no-op conditions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lua/diffview/tests/functional/standard_view_spec.lua	Adds functional coverage for the new panel cursor helpers.
lua/diffview/scene/views/standard/standard_view.lua	Introduces shared save/restore helpers on StandardView.
lua/diffview/scene/views/file_history/listeners.lua	Uses the new helpers on tab_enter/tab_leave instead of inline logic.
lua/diffview/scene/views/diff/listeners.lua	Uses the new helpers on tab_enter/tab_leave instead of inline logic.

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