feat(view)!: replace single_pane_for_one_sided with one_sided_layout

[dlyongemallo]

Boolean becomes value-typed ("default" | "raw", default "default"). "raw" now also substitutes Diff1Raw for diff1_plain bases, not just Diff2. diff1_inline is still skipped.

No migration; the old key is dropped outright.

Closes #184.

[Copilot]

Pull request overview

This PR replaces the boolean config view.single_pane_for_one_sided with a value-typed view.one_sided_layout ("default" | "raw") to control how one-sided diffs (A/?/D) are rendered, extending the “raw/plain buffer” behavior to Diff1 bases (while still skipping diff1_inline).

Changes:

• Introduces view.one_sided_layout config option with schema/type/docs updates and runtime validation.
• Updates FileEntry.with_layout layout selection to substitute Diff1Raw for one-sided entries when one_sided_layout = "raw" (including Diff1 bases; excluding diff1_inline).
• Refreshes functional tests and help docs to match the new option and behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lua/diffview/tests/functional/one_sided_layout_spec.lua	Updates/extends functional coverage for the new view.one_sided_layout behavior and Diff1/Diff2 substitution rules.
lua/diffview/scene/layouts/diff_1_raw.lua	Updates layout documentation to reference the new config key/value.
lua/diffview/scene/file_entry.lua	Adjusts effective layout selection logic to use one_sided_layout and to exclude diff1_inline.
lua/diffview/config.lua	Replaces config schema/type/docs for the new option and adds setup-time validation.
doc/diffview.txt	Replaces the help section for the old boolean key with the new enum option and updated semantics.
doc/diffview_defaults.txt	Updates documented defaults to reflect one_sided_layout.

Comments suppressed due to low confidence (1)

lua/diffview/tests/functional/one_sided_layout_spec.lua:64

• The new view.one_sided_layout validation path (invalid value -> error + fallback to default) isn’t covered by this spec. Adding a small test case here (e.g. set one_sided_layout = "bogus" and assert it resets to "default") would help prevent regressions in the config schema behavior.

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