Add and clarify license

LICENSE

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+This repository contains both software source code and other creative works.
+
+## Code licence
+
+All software source code in this repository is licensed under the MIT License
+below, unless otherwise indicated in a specific file's header.
+
+## MIT License
+
+Copyright (c) 2025 Nathan Cummins
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+## Non-code content licence
+
+All non-code content in this repository — including, but not limited to, music
+(including melodies, harmonies, arrangements, lyrics, sound recordings, and
+other musical elements), audio recordings, images, photographs, written
+biographies, descriptive texts, and project documentation — is copyright © 2025
+Nathan Cummins, all rights reserved, unless otherwise stated in the file itself.
+
+Such non-code content may not be copied, modified, distributed, or otherwise
+used without prior written permission from Nathan Cummins, except where
+explicitly provided under a separate licence or allowed by law.