docs: update v3 migration guide

docs/migration/v3.md

@@ -4,6 +4,30 @@ This page covers every breaking change introduced in **v3.0** and the steps requ
 
 ---
 
+## CRUD
+
+### Facet keys now always use the full relationship chain
+
+In `v2`, relationship facet fields used only the terminal column key (e.g. `"name"` for `Role.name`) and only prepended the relationship name when two facet fields shared the same column key. In `v3`, facet keys **always** include the full relationship chain joined by `__`, regardless of collisions.
+
+=== "Before (`v2`)"
+
+    ```
+    User.status -> status
+    (User.role, Role.name) -> name
+    (User.role, Role.permission, Permission.name) -> name
+    ```
+
+=== "Now (`v3`)"
+
+    ```
+    User.status -> status
+    (User.role, Role.name) -> role__name
+    (User.role, Role.permission, Permission.name) -> role__permission__name
+    ```
+
+---
+
 ## Models
 
 The lifecycle event system has been rewritten. Callbacks are now registered with a module-level [`listens_for`](../reference/models.md#fastapi_toolsets.models.listens_for) decorator and dispatched by [`EventSession`](../reference/models.md#fastapi_toolsets.models.EventSession), replacing the mixin-based approach from `v2`.