licence: SchOS is GPL-2.0-or-later, no longer Apache-2.0

Nexoniarz is the sole author and SchOS was never published under
Apache-2.0, so this is a straight relicence. LICENSE is the FSF GPLv2 text,
every first-party SPDX header moves to GPL-2.0-or-later, and NOTICE,
README.md, docs/licensing.md and the site say so. projects/schfs-linux
follows the rest of the tree.

The GPL gives copyleft: a fork has to ship its source. It also lifts the
old "Linux drivers, never" rule, since GPL-2.0-only code combines with
GPL-2.0-or-later. "Or later" leaves room for GPLv3, but one binary still
has to pick one version, so docs/licensing.md now keeps Apache-2.0 and
GPLv3 code out of the kernel to keep Linux code possible there. BSD
4-clause code goes from "avoid" to "no", because the advertising clause
is GPL-incompatible.

LDLS still ships no .ko. That boundary is now project policy rather than
a legal requirement, and ldls.md, devices.md and licensing.md section 6
say so. refmod declares license=GPL, and ldls-check passes (27 checks).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/Website/download.html

@@ -283,7 +283,7 @@
   <div class="nv-divider"></div>
   <div class="site-footer__row">
     <span class="nv-body">SchOS — kernel <b>SchIFT</b>, filesystem <b>SchFS</b>.</span>
-    <span class="nv-code nv-text-tertiary">Apache-2.0, with the xv6-derived core under MIT.</span>
+    <span class="nv-code nv-text-tertiary">GPL-2.0-or-later, with the xv6-derived core under MIT.</span>
   </div>
 
   <p class="nv-body nv-text-secondary pride-note" id="pride-note">