License: GPL-3.0-or-later

Full GPL-3.0 text in LICENSE, SPDX-License-Identifier header on every
source file, README license section updated. Copyright (C) 2026 Knutwurst.

Tidied a couple of stale comments in patchdl_tile.{c,h} while at it.

src/patchdl_assets.h

@@ -1,3 +1,12 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later
+ * Copyright (C) 2026 Knutwurst
+ *
+ * PatchDL is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option)
+ * any later version. See the LICENSE file in the project root for details.
+ */
+
 #pragma once
 
 #include <stddef.h>