feat(graphql): add JS-safe uint64 marshal helpers

Adds MarshalUint64, UnmarshalUint64, and Uint64Marshaler for gqlgen-
compatible uint64 string scalars without a gqlgen dependency.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -49,6 +49,7 @@ For Postgres, use `dbx.DialectPostgres` and blank-import `dbxp` instead of `dbxm
 | Partial update | `ApplyPtr`, `ApplyComplexPtr`, `ApplyInterfacePtr`, `ApplyVal` |
 | Pointers | `Ptr`, `Val`, `NowPtr`, `TrimPtr`, `TrimPtrToNil`, `IsZero` |
 | Types | `StringKSUID`, `BinaryKSUID`, `JSONB` |
+| GraphQL | `MarshalUint64`, `UnmarshalUint64` |
 
 ## KSUID type selection