chore: relicense from MIT to GPL-2.0-only WITH Classpath-exception-2.0Verified

LICENSE file replaced with the GPLv2-with-Classpath-exception text.
SPDX identifiers updated across every source file (37 *.lisp / *.el),
the CLAUDE.md "License" section, README.md, CHANGELOG.md, and the
funcl.asd license slot.