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/AssetUsageAuditCore/Public/AssetDeletionScan.h

@@ -0,0 +1,151 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "UObject/TopLevelAssetPath.h"
+
+class IAssetRegistry;
+
+/**
+ * What would happen if this set of packages were deleted - worked out before anything is touched.
+ *
+ * Deletion is the one operation in this tool that destroys work, and the audit's own verdicts are
+ * not a safe basis for it on their own: Unknown means "the registry cannot see it", not "unused",
+ * and the whole point of the five-state verdict is that the tool never presents a guess as a fact.
+ * So the scan answers a narrower, checkable question - who still points at this, right now, in the
+ * registry - and leaves the judgement to the person reading it.
+ *
+ * Pure registry and filesystem work. It loads nothing, deletes nothing, and is deliberately
+ * separate from the code that performs the delete: the analysis has to be testable without an
+ * editor, and the actual removal is ObjectTools' job in the Editor module, not ours.
+ */
+namespace AssetDeletionScan
+{
+    /**
+     * Why a ticked package will not be deleted.
+     *
+     * Refusals are policy, not failures, and each one is shown with its reason. Silently dropping a
+     * row from the delete set would be worse than refusing loudly - the user ticked it and is owed
+     * an explanation.
+     */
+    enum class ERefusal : uint8
+    {
+        /** No objection. */
+        None,
+
+        /**
+         * The package is a map.
+         *
+         * Refused by decision, not by capability. A level is the unit the whole tool measures usage
+         * against, and deleting one silently invalidates every other row in the result. The engine
+         * also refuses to delete a level that is currently open, which would make the outcome depend
+         * on which map the user happens to have loaded.
+         */
+        IsLevel,
+
+        /**
+         * A One File Per Actor package - an actor or object belonging to some level.
+         *
+         * Deleting one is not deleting an asset, it is deleting a placed actor out of a map behind
+         * the level editor's back. That belongs in the level editor, with its undo.
+         */
+        IsExternalPackage,
+
+        /** /Engine, /Script or /Temp. Not this project's to delete. */
+        NotProjectContent,
+
+        /** The registry has no asset under this package name; nothing to delete. */
+        Missing
+    };
+
+    ASSETUSAGEAUDITCORE_API const TCHAR* LexToString(ERefusal Refusal);
+
+    /** One line of explanation, suitable for showing beside the row. */
+    ASSETUSAGEAUDITCORE_API FText DescribeRefusal(ERefusal Refusal);
+
+    /** One ticked package and what stands in the way of deleting it. */
+    struct FCandidate
+    {
+        FName PackageName;
+        FName AssetName;
+        FTopLevelAssetPath ClassPath;
+
+        /** Content/... form, as the report shows it. */
+        FString PathFromProjectRoot;
+
+        ERefusal Refusal = ERefusal::None;
+
+        /**
+         * Packages that reference this one and are NOT themselves being deleted.
+         *
+         * The exclusion matters: deleting a Blueprint together with the mesh only it uses is a
+         * clean operation, and listing the Blueprint as a blocker would make every sensible
+         * multi-asset delete look dangerous. Truncated to FOptions::MaxReferencersListed - the list
+         * is there to be read, and forty names is not read.
+         */
+        TArray<FName> OutsideReferencers;
+
+        /** Full count before truncation. */
+        int32 OutsideReferencerCount = 0;
+
+        /** How many of those referencers are maps or OFPA packages - i.e. it is placed in a level. */
+        int32 LevelReferencerCount = 0;
+
+        /**
+         * The file exists on disk and is marked read-only.
+         *
+         * Worth surfacing here rather than discovering it as a failure afterwards: this project is
+         * Perforce-primary and unopened files are read-only by default, so a delete attempted
+         * without checking out first fails per-file, halfway through.
+         */
+        bool bReadOnlyOnDisk = false;
+
+        bool CanDelete() const
+        {
+            return Refusal == ERefusal::None;
+        }
+    };
+
+    struct FOptions
+    {
+        /** How many referencer names to keep per candidate. The count is always exact. */
+        int32 MaxReferencersListed = 8;
+    };
+
+    // Exported: Summarise is defined out of line, so the Editor module cannot link without this.
+    struct ASSETUSAGEAUDITCORE_API FStats
+    {
+        int32 Requested = 0;
+        int32 Deletable = 0;
+
+        int32 RefusedLevels = 0;
+        int32 RefusedExternal = 0;
+        int32 RefusedNotProjectContent = 0;
+        int32 RefusedMissing = 0;
+
+        /** Deletable candidates that something outside the set still points at. */
+        int32 WithOutsideReferencers = 0;
+
+        /** Deletable candidates whose file is read-only on disk. */
+        int32 ReadOnly = 0;
+
+        FString Summarise() const;
+    };
+
+    /**
+     * Classify the packages and collect their outside referencers.
+     *
+     * Order is preserved, refusals included, so the caller can show the whole ticked set with a
+     * reason against each row instead of a shorter list that quietly lost entries.
+     *
+     * Referencers are queried with AssetUsageAudit::MakeTraversalQuery() for the same reason the
+     * audit uses it: a Hard-only query misses every soft reference, and a soft reference still
+     * breaks when its target disappears - it just breaks at runtime instead of at load.
+     */
+    ASSETUSAGEAUDITCORE_API TArray<FCandidate> Scan(
+        IAssetRegistry& Registry,
+        const TArray<FName>& Packages,
+        const FOptions& Options,
+        FStats& OutStats);
+}