feat(backend): override create_schema_differ() to supply SQLServerSchemaDiffer

The core migrated schema-diff resolution to the dependency-inversion hook
SQLDialectBase.create_schema_differ() and no longer imports concrete
backend differs. Override the hook so SQL Server keeps its ordinal-position-aware
column comparison in migration snapshot diffs.

- Implement SQLServerDialect.create_schema_differ() returning SQLServerSchemaDiffer
- Add changelog fragment

Author: vistart

changelog.d/53.added.md

@@ -0,0 +1 @@
+Added `SQLServerDialect.create_schema_differ()` returning `SQLServerSchemaDiffer` so the dialect exposes its ordinal-position-aware schema comparison through the core's new dependency-inversion hook (`SQLDialectBase.create_schema_differ()`), keeping migration snapshot diffs correct now that the core no longer imports concrete backend differs.

src/rhosocial/activerecord/backend/impl/sqlserver/dialect.py

@@ -371,6 +371,14 @@ def get_server_version(self) -> Tuple[int, int, int]:
         """Return the SQL Server version this dialect is configured for."""
         return self.version
 
+    def create_schema_differ(self):
+        """Return the SQL Server schema differ (ordinal-position aware)."""
+        from rhosocial.activerecord.backend.impl.sqlserver.schema.differ import (
+            SQLServerSchemaDiffer,
+        )
+
+        return SQLServerSchemaDiffer()
+
     def format_date_trunc_expression(self, expr: "Any") -> Tuple[str, Tuple]:
         source_sql, source_params = expr.source.to_sql()
         field = expr.field.value.upper()