feat: asset deletion scan and delete dialog (WIP)

Adds AssetDeletionScan to Core (no UI/editor-pipeline deps, invariant holds)
and SAssetDeleteDialog to the Editor module, plus 10 spec cases.

Co-Authored-By: Claude Code <noreply@anthropic.com>

Source/AssetUsageAuditEditor/Private/SAssetUsageAuditPanel.h

@@ -278,6 +278,31 @@ private:
     FReply OnExportFilesClicked();
     bool HasCheckedAssets() const;
 
+    // --- Deleting ----------------------------------------------------------------------------
+
+    /**
+     * Delete the ticked assets, after two confirmations.
+     *
+     * Sits beside Export because it acts on the same ticked set, and is the reason the audit is
+     * worth running: the point of finding unused content is to be able to remove it. It is kept
+     * visually apart and never becomes the default action.
+     *
+     * Levels are refused outright - see AssetDeletionScan::ERefusal::IsLevel. Everything else the
+     * window explains before anything is touched, and the destruction itself is ObjectTools' code,
+     * not ours.
+     */
+    FReply OnDeleteClicked();
+
+    /**
+     * Drop deleted packages from the result without re-running the sweep.
+     *
+     * A full re-run after a delete would cost minutes on this project, and the rows that remain are
+     * still correct - what changed is that some assets are gone, plus the referencer counts of
+     * whatever pointed at them. The result is marked stale so the numbers are not mistaken for a
+     * fresh sweep.
+     */
+    void ForgetDeletedPackages(const TArray<FName>& DeletedPackages);
+
     /** Byte copy of the package files, expanding levels to their OFPA packages. */
     void RunCopyExport(const TArray<FAssetUsageRow>& Rows, const FAssetExportRequest& Request);