Incorrect +spdx

Tail of special meta +spdx should be SPDX-compliant headers.

Incorrect:

+spdx foo bar

# Foo.
[] > foo

Correct:

+spdx SPDX-License-Identifier: MIT

# Foo.
[] > foo

In XMIR, spdx meta should look like this:

<metas>
  <meta>
    <head>spdx</head>
    <tail>SPDX-License-Identifier: MIT</tail>
    <part>SPDX-License-Identifier:</part>
    <part>MIT</part>
  </meta>
  <meta>
    <head>spdx</head>
    <tail>SPDX-FileCopyrightText Copyright (c) 2016-2025 Objectionary.com</tail>
    <part>SPDX-FileCopyrightText</part>
    <part>Copyright</part>
    <part>(c)</part>
    <part>2016-2025</part>
    <part>Objectionary.com</part>
  </meta>
</metas>

Please note, that these two contain valid SPDX headers. Thus, you can use any number of SPDX-compliant headers.