fix(fold): skip foldmethod=diff override on solo-content side of added/deleted files

[dlyongemallo]

Fixes #299.

[Copilot]

Pull request overview

This PR fixes issue #299, where code folding was broken on the content side of added or deleted files. In a two-way diff of a new or deleted file, one pane is nulled, so foldmethod=diff produces no useful folds on the pane that actually has content. The fix detects when the paired window in the layout is nulled and, in that case, leaves the buffer's own folding (ftplugin/treesitter/ufo/etc.) intact instead of forcing diffview's diff-fold winopts.

Changes:

• Added Window:_paired_side_is_null() to detect when a sibling window in the parent layout is nulled.
• Skips the view.foldlevel override and passes skip_fold into apply_file_winopts on the solo-content side, filtering out foldmethod/foldenable/foldlevel so the buffer's own folding survives.
• Added two regression tests covering the nulled-sibling (skip) and non-nulled (apply) cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lua/diffview/scene/window.lua	Adds nulled-sibling detection, skip_fold filtering in apply_file_winopts, and gates the foldlevel override on the solo-content side.
lua/diffview/tests/functional/window_spec.lua	Adds regression tests verifying fold options are preserved when a sibling is nulled and still applied otherwise.

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