Add enum types valuemust be the same across source schemas

spec/Section 4 -- Composition.md

@@ -14,6 +14,73 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Enum Type Values Must Be The Same Across Source Schemas
+
+**Error Code**
+
+ENUM_VALUES_MUST_BE_THE_SAME_ACROSS_SCHEMAS
+
+**Formal Specification**
+
+- Let {enumNames} be the set of all enum type names across all source schemas.
+- For each {enumName} in {enumNames}:
+  - Let {enums} be the list of all enum types from different source schemas with the name {enumName}.
+  - {EnumsAreMergeable(enums)} must be true.
+
+EnumsAreMergeable(enums):
+
+- Let {inaccessibleValues} be the set of values that are declared as `@inaccessible` in {enums}.
+- Let {requiredValues} be the set of values in {enums} that are not in {inaccessibleValues}.
+- For each {enum} in {enums}
+  - Let {enumValues} be the set of all values of {enum} that are not in {inaccessibleValues}.
+  - {requiredValues} must be equal to {enumValues}
+
+**Explanatory Text**
+
+This rule ensures that enum types with the same name across different source schemas in a composite schema have identical sets of values. 
+Enums must be consistent across source schemas to avoid conflicts and ambiguities in the composite schema.
+
+When an enum is defined with differing values, it can lead to confusion and errors in query execution. 
+For instance, a value valid in one schema might be passed to another where it's unrecognized, leading to unexpected behavior or failures. 
+This rule prevents such inconsistencies by enforcing that all instances of the same named enum across schemas have an exact match in their values.
+
+In this example, both source schemas define `Enum1` with the same value `BAR`, satisfying the rule:
+
+```graphql example
+enum Enum1 {
+  BAR
+}
+
+enum Enum1 {
+  BAR
+}
+```
+
+Here, the two definitions of `Enum1` have different values (`BAR` and `Baz`), violating the rule:
+
+```graphql counter-example
+enum Enum1 {
+  BAR
+}
+
+enum Enum1 {
+  Baz
+}
+```
+
+Here, the two definitions of `Enum1` have shared values and additional values declared as `@inaccessible`, satisfying the rule:
+
+```graphql example
+enum Enum1 {
+  BAR
+  BAZ @inaccessible
+}
+
+enum Enum1 {
+  BAR
+}
+``` 
+
 ### Merge
 
 ### Post Merge Validation