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/SAssetDeleteDialog.h

@@ -0,0 +1,46 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+
+/** What the delete window ended up doing. Everything the panel needs to report afterwards. */
+struct FAssetDeleteOutcome
+{
+    /** Packages handed to the window. */
+    int32 Requested = 0;
+
+    /** Refused by policy - levels, OFPA packages, engine content. Never reaches the engine. */
+    int32 Refused = 0;
+
+    /** Ticked and passed to ObjectTools. */
+    int32 Attempted = 0;
+
+    /**
+     * Packages the registry can no longer find afterwards.
+     *
+     * Measured rather than taken from the engine's return value: ObjectTools reports a count of
+     * deleted *objects*, and one package can hold several. Asking the registry what is actually
+     * gone is the only figure that matches what the user will see in the Content Browser.
+     */
+    TArray<FName> DeletedPackages;
+
+    /** True when the user closed or cancelled the window; nothing was touched. */
+    bool bCancelled = true;
+};
+
+/**
+ * Show the delete window: what is about to be destroyed, what still points at it, and why some
+ * rows are refused.
+ *
+ * Modal, and deliberately heavier than the export window. Export is reversible by deleting the
+ * output folder; this is not, and the window's job is to be read rather than clicked through -
+ * hence the per-row referencer count, the explicit acknowledgement, and Cancel as the default.
+ *
+ * The window does not delete anything itself. Once confirmed it hands the assets to
+ * ObjectTools::DeleteAssets with the engine's own confirmation enabled, so the familiar Delete
+ * Assets dialog - referencer list, Force Delete, Replace References - appears exactly as it does
+ * from the Content Browser. Reimplementing that would mean reimplementing reference replacement,
+ * and getting it subtly wrong on someone's project.
+ */
+FAssetDeleteOutcome ShowAssetDeleteDialog(const TArray<FName>& Packages);