Add spec for Long custom scalar type

scalars/contributed/jakobmerrild/long.md

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+<!-- cspell:ignore jakobmerrild -->
+
+# Long — GraphQL Custom Scalar
+
+Author - jakobmerrild
+
+Date - 2025-12-18
+
+**License and Copyright**
+
+Copyright © GraphQL contributors. This specification is licensed under
+[OWFa 1.0](https://www.openwebfoundation.org/the-agreements/the-owf-1-0-agreements-granted-claims/owfa-1-0).
+
+# Overview
+
+This scalar represents a 64-bit signed integer (non-fractional) value, ranging
+from `-2^63` to `2^63-1`.
+
+# Name
+
+The scalar should be named `Long` to match commonly used names for the same data
+structure in a variety of programming languages. Alternatively the scalar can be
+named `Int64` to represent the 64-bit encoding.
+
+# Result spec
+
+A field of type `Long` should result in a `string` value representing a base-10
+encoding of the underlying value without a fractional or exponential part. A
+leading `-` should only be added if the field represents a negative value.
+
+These are valid examples:
+
+| Output                   | Explanation                                                    |
+| ------------------------ | -------------------------------------------------------------- |
+| `"0"`                    | Zero is a valid integer within the range                       |
+| `"-9223372036854775808"` | This is the lowest value that can be represented in the range  |
+| `"9223372036854775807"`  | This is the largest value that can be represented in the range |
+
+These are invalid examples:
+
+| Output                    | Explanation                                                              |
+| ------------------------- | ------------------------------------------------------------------------ |
+| `"+1234"`                 | Leading `+` is not allowed                                               |
+| `"-10223372036854775808"` | Value is lower than `-2^63`                                              |
+| `"12223372036854775807"`  | Value is greater than `2^63-1`                                           |
+| `"123.0"`                 | Fractional part is not allowed even if it is zero                        |
+| `"1e6"`                   | Exponential notation is not allowed, even if it represents a valid value |
+| `12345`                   | Number representations of a valid value are not allowed                  |
+
+# Input spec
+
+For input `string` values shall be accepted so long as they fall within the
+range represented by the spec.
+
+These are valid examples:
+
+| Input                    | Explanation                                                    |
+| ------------------------ | -------------------------------------------------------------- |
+| `"0"`                    | StringValue within range                                       |
+| `"-9223372036854775808"` | This is the lowest value that can be represented in the range  |
+| `"9223372036854775807"`  | This is the largest value that can be represented in the range |
+
+These are invalid examples:
+
+| Input                     | Explanation                                                                                                                  |
+| ------------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
+| `"-10223372036854775808"` | Value is lower than `-2^63`                                                                                                  |
+| `"12223372036854775807"`  | Value is greater than `2^63-1`                                                                                               |
+| `"123.0"`                 | FloatValue is not allowed                                                                                                    |
+| `"FFFFF"`                 | A StringValue containing a base-16 representation of a valid value is not allowed                                            |
+| `"6543.000"`              | A StringValue containing a base-10 representation with a fractional part is not allowed. Even if the fractional part is zero |
+| `987654321`               | An IntValue containing a base-10 representation of the value is not allowed                                                  |