Documents: closing a modified background tab keeps the document in front

Closing a modified tab that is not in front shows it while the Save
Changes dialog asks about it. After Save or Don't Save, the closed tab's
neighbor became the current document instead of the document that was in
front before the question.

Closing a background tab with its close button, or `file.close` with its
`index`, now remembers the uid of the document in front in the dialog's
protected `__front` field, and once the asked document closes, that
document is made current again while it is still open. The field
survives a Save dialog shown off the UI thread. Cancel keeps the asked
document in front, closing the document in front still brings its
neighbor forward, and Close All and Quit are unchanged.

docs/target-app-parity.md

@@ -918,6 +918,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - The canvas context menu lists Object › Path › Smooth… while the selection holds a path, in a group or a compound path too. It opens the Smooth slider, as the Object menu does (`right_click_on_a_path_opens_the_smooth_slider`, `smooth_follows_the_selections_paths`).
   - Native macOS save sheets offer a localised Format menu (#866), built from the request's existing format filters. It selects the suggested filename's format and updates the suffix when changed, including legacy `.drawcraft` names. `apps/vectorcraft/src/mac_save_format.rs` has filename regressions; an actual native SVG save, selection of PDF and cancellation without changing the saved document were verified.
   - Save / Don't Save / Cancel before closing or quitting with unsaved documents (tabs, Close, Close All, Quit, window close).
+  - Closing a modified tab that is not in front shows it while asking to save; after Save or Don't Save the document that was in front before the question is in front again, and Cancel keeps the asked document in front. Tests: `closing_a_modified_background_tab_keeps_the_front_document_after_dont_save`, `closing_a_modified_background_tab_keeps_the_front_document_after_save`, `cancel_keeps_the_asked_background_tab_in_front` and `save_changes_answered_by_a_save_dialog_keeps_the_front_document`.
   - Four brightness themes, persistent preferences.
   - **Snapping quick access (#6164):** Control bar › Snapping includes Snap to Glyph (`view.snapToGlyph`) and Alignment Guides (`prefs.set`, `alignmentGuides`). The headless pointer/accessibility test `the_control_bar_snapping_menu_shares_glyph_and_alignment_settings` covers both commands, independent state, checked states after reopening, and all eight menu rows within a 1200 × 400-point window. Floating-panel parity remains unverified; scores and estimates unchanged.
   - **Pixel Snapping Options:** Control bar › Snapping › Pixel Snapping Options… sets whether View › Snap to Pixel snaps drawing, moving and scaling, each on by default (`view.snapToPixel.options`, the `pixelSnap` preference group; [the help page](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/draw-pixel-perfect-art.html)). Tests: `pixel_snapping_options_read_set_and_reset`, `pixel_snapping_options_choose_the_edits_snap_to_pixel_rounds`, `handle_drag_follows_the_while_scaling_option`, `move_follows_the_while_moving_option`, `an_anchor_drag_follows_the_while_moving_option`, `new_ruler_guide_follows_the_while_moving_option` and `the_control_bar_snapping_menu_opens_pixel_snapping_options`. Scores and estimates unchanged.