Make a folder LICENSE file match the per-file SPDX headers when a directory mixes MIT and GPL files

Review finding from a repo where one folder holds files under different licences (some MIT, one GPL because it derives from GPL code). The folder's LICENSE preamble named only a few MIT files and said 'the rest is GPL'. Later commits added new MIT-headed files (C sources, a JS helper, SystemVerilog testbench files). The preamble then contradicted their SPDX headers, and the new files weren't named at all.

Fix that avoids the drift: - Make the per-file SPDX-License-Identifier header the source of truth. - Word the LICENSE preamble by rule, not by list, for example: 'The MIT terms below apply to every file in this folder whose SPDX header says MIT, and to the libraries built from them; files whose header says GPL-2.0-or-later are under the GPL terms that follow.' - Don't relicense anything as part of the fix; only align the description with the headers. - Add a check that greps every file for an SPDX line and fails if one is missing or names a licence the LICENSE file doesn't describe.

Also worth a privacy grep before publishing: test variable names can accidentally match private naming patterns (course or project codes). Rename them in a new commit rather than rewriting history.