feat(file-panel): add multi-file selection for batch stage/unstage/restore

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Adds multi-file selection support to Diffview’s FilePanel, enabling batch stage/unstage and restore operations from the panel UI.

Changes:

• Introduces selection state and APIs on FilePanel (toggle_selection, is_selected, get_selected_files, clear_selections) plus functional tests for this behavior.
• Renders a visual marker for selected files in the file panel and adds a highlight group link for it.
• Adds a default keybinding (w) to toggle selection and updates staging/restoring listeners to operate on selected files when present.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lua/diffview/tests/functional/panel_spec.lua	Adds functional coverage for FilePanel selection APIs.
lua/diffview/scene/views/diff/render.lua	Displays a marker for selected files during file panel rendering.
lua/diffview/scene/views/diff/listeners.lua	Implements multi-select toggle action and batch stage/restore behavior when selections exist.
lua/diffview/scene/views/diff/file_panel.lua	Adds selection state tracking and exposes selection-related methods.
lua/diffview/hl.lua	Adds highlight link for the “marked/selected” indicator.
lua/diffview/config.lua	Adds default keymap (w) for toggling selection in the file panel.
lua/diffview/actions.lua	Exposes toggle_select_entry as an action name.
doc/diffview_defaults.txt	Documents the new default keymap.

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