Default value compatibility validation is only specified for Object field arguments

[PR #793](https://github.com/graphql/graphql-spec/pull/793) (Default value validation & coercion, September 2025 edition) added this rule to **Objects → Type Validation**:

> 5\. If the argument has a default value it must be compatible with {argumentType} as per the coercion rules for that type.

However, the parallel rule is absent from the other locations where default values can be defined:

- **Interfaces → Type Validation**: interface field arguments have no default-value compatibility rule (the argument rule list ends at `IsInputType`).
- **Input Objects → Type Validation**: input fields have no compatibility rule for their default values (the section did gain the `InputObjectDefaultValueHasCycle` rule, but nothing requiring the default itself to be compatible with the field type).
- **Type System Directives → Type Validation**: directive definition arguments have no default-value compatibility rule.

The reference implementation validates all four locations: [graphql-js#3814](https://github.com/graphql/graphql-js/pull/3814) calls `validateDefaultValue` from `validateFields` (which runs for both Object and Interface types), `validateInputFields`, and `validateDirectives` — and removed a `// TODO: validate default values?` marker in the interface-implementation checks.

If the omission is unintentional, the fix seems editorial: add the same bullet to the Interfaces, Input Objects, and Type System Directives validation sections.

_Drafted by Claude (Anthropic AI assistant)._

Start with the Interfaces, Input Objects, and Type System Directives Type Validation sections named in the issue, then compare the corresponding validation behavior described in graphql-js#3814. Done means each listed location specifies that a default value must be compatible with its type according to the coercion rules.

Written by the indexing model from the issue text.