Menus: Delete Empty Artboards' enabled check does not walk the document every frame

The Artboards panel menu asks whether Delete Empty Artboards can run on
every frame while it is open, and each ask measured the document. With
an open path of 200,000 anchors across two artboards, that took 70.4 ms
a frame in a release build.

The answer is now kept per open document with the version of the
document it was measured on, and is measured again when the document
changes. A later frame takes under 0.001 ms. The first frame after an
edit measures once. A change of the selection or the view keeps the
answer.

docs/target-app-parity.md

@@ -559,6 +559,7 @@ since 2026-10-01.
 - **Selected artboard deletion:** the Artboards panel menu, panel trash button and Properties use the same selected set; selecting all retains artboard 1. Pointer-event tests cover nonadjacent and single selections, the original first-board survivor, ordinary artwork, Undo/Redo and the single-board guard. Public reference: [Delete artboards](https://helpx.adobe.com/illustrator/desktop/create-manage-artboards/add-edit-artboards/delete-artboards.html).
 - **Delete Empty Artboards:** the Artboards panel menu item ([Delete artboards](https://helpx.adobe.com/illustrator/desktop/create-manage-artboards/add-edit-artboards/delete-artboards.html)) runs `artboard.deleteEmpty`: every unlocked artboard with no artwork goes with its guides in one undo step, artboard 1 stays when every artboard is empty and none is locked, and the panel makes an artboard that is left active; an artboard is empty when no object (hidden, locked and template art included; guides not) has visual bounds that meet or touch it, a group counting by its members and a clip group or clipping layer by its members inside the clip (our reading of "no artwork"; the pages give no finer rule). Engine tests in `tests/panelcmds.rs` (`delete_empty_artboards_*`) and the panel test `delete_empty_artboards_deletes_the_empty_ones_and_selects_one_left`.
   The menu item's enabled check measures nothing in a document with one artboard, and Save Gradient Preset's check reads the proxy's paint without measuring the object; both give the same answer as before. An artboard that no path's control points, grown by the farthest its strokes reach, can meet is found empty without measuring the path. With an open path of 200,000 anchors selected, the checks take under 0.01 ms instead of 21.9 ms and 17.2 ms in a release build; with a second, empty artboard, Delete Empty Artboards' check takes 0.85 ms instead of 42.1 ms. Tests `delete_empty_artboards_is_enabled_exactly_when_it_deletes_something`, `delete_empty_artboards_enabled_state_matches_the_command_on_varied_documents`, `empty_artboard_checks_match_the_visual_bounds_on_varied_strokes`, `the_empty_artboard_check_matches_measuring_every_artboard`, `the_save_check_matches_saving_with_and_without_a_gradient`, `the_unmeasured_proxy_paint_is_the_measured_one` and `the_unmeasured_proxy_paint_is_the_measured_one_on_varied_objects`.
+  Delete Empty Artboards' enabled check measures each version of a document once, so the panel menu does not measure the document again on every frame while it is open. With an open path of 200,000 anchors across two artboards, the check takes under 0.001 ms on a later frame instead of 70.4 ms in a release build, and with 20,000 objects on two artboards it takes under 0.001 ms instead of 9.1 ms. Tests `delete_empty_artboards_check_measures_a_document_once`, `delete_empty_artboards_check_follows_each_change_to_the_document`, `delete_empty_artboards_check_follows_a_change_in_place`, `delete_empty_artboards_check_follows_a_document_switch` and `delete_empty_artboards_check_matches_measuring_after_varied_changes`.
   The Eyedropper, the Live Paint check for paint to sample, the Appearance panel's Characters chips and the colors the Libraries panel adds from the selection also read the proxy's paint without measuring the object; the Eyedropper measures it only to sample a gradient or a pattern, and the Libraries panel only for a gradient. The colors and paints are the same as before. With an open path of 200,000 anchors, the Live Paint check and the Eyedropper's sample for a point inside it take 0.004 ms together instead of 26.0 ms in a release build. Tests `samples_give_the_measured_proxy_paints_colors` and `colors_and_gradients_come_from_compound_paths_groups_and_colorized_objects`.
 - **Artboard history selection:** headless pointer/key tests in `panels/properties.rs` pass for panel New → Undo → Delete/Backspace/Properties, Duplicate → Undo → Delete, pruning removed indices and preserving valid multiselection. They assert board survivors, artwork and object selection, Move/Scale Artwork options, saved state and Undo/Redo. Full workspace CI passes.
 - **Artboard tab-switch follow-up:** changing the active document through its tab aligns the Artboard tool with the displayed board before Properties Delete; clicking the already-active tab preserves multiselection and Move/Scale Artwork options. Three headless `chrome::tests::artboard_document_tab_*` regressions cover actual tab A → B → A clicks followed by Properties Delete, surviving boards and guides, preserved artwork/symbols and Undo/Redo; controls cover same-tab multiselection, Move/Scale Artwork options and other tools. Historical focused validation on `dbd2497b` recorded one expected RED failure with two passing controls, then all three GREEN tests passed. All three also passed in historical full CI on `a0671777`. A full CI retry on the five-unit combined source at `a0671777` passed all seven stages with 6,613 test outcomes on 2026-10-14. An earlier full-CI run on the identical source panicked in the randomized `ai_mutated_save_never_panics` test; the cause remains unresolved. This five-fix batch does not claim to repair that failure.