refactor(layout): pull out some repeated code into their own functions

[dlyongemallo]

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[Copilot]

Pull request overview

This PR refactors diff layout implementations to reduce duplicated logic by centralizing common file-assignment and window-creation behavior in the base Layout, and adds functional coverage for the new shared helpers.

Changes:

• Introduces Layout:set_file_for(), a default Layout.use_entry(), and Layout.create_wins() to consolidate repeated per-layout logic.
• Adds symbols declarations to Diff1–Diff4 and updates concrete layouts to use create_wins() instead of bespoke window split code.
• Expands functional tests to cover symbols, set_file_for(), and create_wins() (mocked + integration with real splits).

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lua/diffview/scene/layout.lua	Adds shared helpers (set_file_for, default use_entry, create_wins) to eliminate repeated logic in concrete layouts.
lua/diffview/scene/layouts/diff_1.lua	Defines symbols and switches layout creation to create_wins().
lua/diffview/scene/layouts/diff_2.lua	Defines symbols and removes now-redundant per-layout use_entry/file setter logic.
lua/diffview/scene/layouts/diff_2_hor.lua	Refactors window creation to create_wins() for horizontal 2-way layout.
lua/diffview/scene/layouts/diff_2_ver.lua	Refactors window creation to create_wins() for vertical 2-way layout.
lua/diffview/scene/layouts/diff_3.lua	Defines symbols and removes now-redundant per-layout use_entry/file setter logic.
lua/diffview/scene/layouts/diff_3_hor.lua	Refactors window creation to create_wins() for horizontal 3-way layout.
lua/diffview/scene/layouts/diff_3_mixed.lua	Refactors window creation to create_wins() for mixed 3-way layout (non-trivial creation order).
lua/diffview/scene/layouts/diff_3_ver.lua	Refactors window creation to create_wins() for vertical 3-way layout.
lua/diffview/scene/layouts/diff_4.lua	Defines symbols and removes now-redundant per-layout use_entry/file setter logic.
lua/diffview/scene/layouts/diff_4_mixed.lua	Refactors window creation to create_wins() for mixed 4-way layout (non-trivial creation order).
lua/diffview/tests/functional/layouts_spec.lua	Adds functional tests for symbols, set_file_for(), and create_wins() (mock + real-window integration).

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