fix(panel): preserve selections and folder state across tab switches

[dlyongemallo]

Partially relates to sindrets#582.

[Copilot]

Pull request overview

This PR preserves file selections and directory fold states across tab switches and file list refreshes in the diffview panel. Previously, selections were keyed by object identity and cleared on every update; now they use stable string keys (kind:path) and are pruned rather than cleared.

Changes:

• Selections now use stable kind:path string keys instead of object identity, with prune_selections() replacing clear_selections() on file updates
• Directory collapsed state is propagated to underlying tree nodes via set_dir_collapsed(), fixing flattened directory state persistence; _node in flattened dirs now points to the outermost node
• New configurable signs for selected files/dirs and directory selection state rendering in the panel

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lua/diffview/scene/views/diff/file_panel.lua	Add stable selection keys, select_file/deselect_file/prune_selections/dir_selection_state/set_dir_collapsed methods
lua/diffview/scene/views/diff/listeners.lua	Use new selection/collapse APIs; suppress highlight on tab enter; prune instead of clear
lua/diffview/scene/views/diff/diff_view.lua	Three-valued highlight parameter (true/false/nil); pass nil instead of false on update
lua/diffview/scene/views/diff/render.lua	Thread conf through render functions; render dir selection indicators
lua/diffview/ui/models/file_tree/file_tree.lua	Fix _node to point to flatten root in flattened directory chains
lua/diffview/config.lua	Add selected_file, selected_dir, partially_selected_dir sign defaults
doc/diffview.txt	Document new signs and updated selection behavior
doc/diffview_defaults.txt	Add new sign defaults
lua/diffview/tests/functional/panel_spec.lua	Comprehensive tests for new selection and collapse behavior

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