feat(project-system): recover malformed element files without ever rewriting them

docs/specs/001-agent-editing-toolkit/contracts/mcp-tools.md

@@ -97,7 +97,7 @@ A `session` may be **file-opened** (headless server) or **live** (in-app host bo
 
 ### `open_project`
 - **Input**: `{ "path": string }` (read — unrestricted).
-- **Output**: `{ "session": string, "source": "File", "summary": { scenes, elements, duration, frameSize } }`.
+- **Output**: `{ "session": string, "source": "File", "summary": { scenes, elements, duration, frameSize }, "warnings": string[], "recoveryIncidents": [{ "sceneId": string, "sceneName": string, "elementFile": string, "reason": "TypeNotFound" | "DeserializationFailed", "typeName": string | null, "message": string | null }] }`. `warnings` remains the presentation-oriented recovery summary; `recoveryIncidents` exposes the same incidents as stable structured data. `sceneId` and `sceneName` identify the containing scene when different scenes contain the same relative element path. `elementFile` is a forward-slash scene-relative path when available, otherwise the element name. `typeName` is the original serialized discriminator when available, otherwise `null`; `message` is the unmodified nullable deserialization error rather than a presentation fallback.
 - **Errors**: `media_not_found`, `schema_version_mismatch` (project written by an incompatible schema — surfaced, not silently dropped, per FR-031/FR-013).
 
 ### `attach_active_editor` *(in-app host only)*

src/Beutl.AgentToolkit/Documents/DeclarativeDocumentApplier.cs

@@ -220,7 +220,11 @@ private void ApplyKeyFrame(IKeyFrame keyFrame, JsonObject desired)
 
         if (desired.TryGetPropertyValue(nameof(KeyFrame.Easing), out JsonNode? easingNode) && easingNode is not null)
         {
-            keyFrame.Easing = DeserializeEasing(easingNode);
+            Easing desiredEasing = DeserializeEasing(easingNode);
+            if (!AreEquivalentEasings(keyFrame.Easing, desiredEasing))
+            {
+                keyFrame.Easing = desiredEasing;
+            }
         }
 
         if (desired.TryGetPropertyValue(nameof(KeyFrame.KeyTime), out JsonNode? keyTimeNode) && keyTimeNode is not null)
@@ -823,6 +827,21 @@ private static Easing DeserializeEasing(JsonNode node)
             "Easing must be a type string or spline object."));
     }
 
+    private static bool AreEquivalentEasings(Easing current, Easing desired)
+    {
+        if (current.GetType() != desired.GetType())
+        {
+            return false;
+        }
+
+        return current is not SplineEasing currentSpline
+               || desired is SplineEasing desiredSpline
+               && currentSpline.X1 == desiredSpline.X1
+               && currentSpline.Y1 == desiredSpline.Y1
+               && currentSpline.X2 == desiredSpline.X2
+               && currentSpline.Y2 == desiredSpline.Y2;
+    }
+
     internal static string? ValidateEasingNode(JsonNode? node)
     {
         if (node is JsonValue value && value.TryGetValue(out string? typeName))