Desktop tools can't operate a Save sheet: target could not be revalidated before mutation #378

Found in study scenario 20 (#218) on 0.0.36, natembp.

Steps: @ace Open TextEdit, create a new document, type "Ace study scenario 20", save it to ~/Desktop/ace-study-20.txt, …, with the channel's desktop tools on.

Observed:

• desktop_launch, desktop_menu and desktop_insert worked: the text was typed and the document converted to plain text.
• Once the Save sheet opened, every mutation failed: "The native Accessibility target could not be revalidated before mutation." The caption stayed on "Saving TextEdit document to Desktop location" for about 2 minutes.
• The agent then fell back to osascript, which raised the macOS Automation prompt ("Ace Canary" wants access to control "TextEdit"). That prompt can't be answered hands-off, so the run ended blocked.
• No file was written.

Likely area: Peekaboo's mutation planner (DesktopTargetPlanning+MutationPlanners, DialogService+PreparedActions) when the target is a sheet attached to a document window. The sheet's identity or window probably changes between the snapshot and the mutation.

Expected: Save and Open sheets are among the most common native dialogs; desktop tools should be able to fill in a name and location and press Save. Recheck by rerunning scenario 20 step 3.