feat(compiler): plan pure enum widening as a supported ExtendEnum step

Adding variants to an enum property previously planned as an unsupported
property-type change (OG-MF-106): enum values live inside PropType and
the planner compared whole types. Special-case the one safe shape — same
scalar/list/nullability and the desired value set a strict superset of
the accepted one — into a new ExtendEnum step carrying the added values.
Narrowing, variant renames, and enum<->String conversions still plan as
OG-MF-106; a pure reorder was already a no-op (the schema IR sorts +
dedups enum values).

Planner matrix pinned in in-source tests (widening, reorder-as-no-change,
narrowing, rename, widen+nullability-flip, enum<->String both ways); CLI
plan rendering gains the ExtendEnum arm.

crates/omnigraph-cli/src/output.rs

@@ -566,6 +566,19 @@ pub(crate) fn render_schema_plan_step(step: &SchemaMigrationStep) -> String {
             schema_type_kind_label(*type_kind),
             type_name
         ),
+        SchemaMigrationStep::ExtendEnum {
+            type_kind,
+            type_name,
+            property_name,
+            added_values,
+        } => format!(
+            "extend enum '{}.{}' (+{}) on {} '{}'",
+            type_name,
+            property_name,
+            added_values.join(", +"),
+            schema_type_kind_label(*type_kind),
+            type_name
+        ),
         SchemaMigrationStep::UpdateTypeMetadata {
             type_kind,
             name,