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>

kernel/drivers/bus/xhci/xhci.c

@@ -10,9 +10,9 @@
 // the controller is not complicated; BSD is much more valuable as the
 // reference you check yourself against than as the code you paste.
 //
-// (Licensing points the same way. The BSD stacks are BSD-2/3-clause, which is
-// compatible with Apache-2.0 -- Linux's drivers are GPL-2.0-only, which is
-// not. See docs/guides/adding-a-driver.md.)
+// (Licensing no longer decides it: the BSD stacks are BSD-2/3-clause and
+// Linux's drivers are GPL-2.0-only, and both combine with GPL-2.0-or-later.
+// The argument above is the whole reason. See docs/licensing.md.)
 //
 // Enumeration polls the event ring instead of using interrupts. That is not
 // laziness: pci_bind_all() runs from kmain() with interrupts disabled and no