This pull-request adds explicit MIT SPDX headers to the two files adapted from fixest project and checks in the written permission that allows us to re-license those GPL-origin snippets.

No functional code has changed; this is purely a licensing/compliance cleanup.

@lrberge, if you are ok with the re-license of the files derived from fixest, could you please confirm permission by replying to this PR with the sentence below (or similar, if you prefer, but it must include the same substance)? This reply will be archived in the repo under THIRD_PARTY_PERMISSIONS.md:

I, Laurent Berge, hereby grant irrevocable permission to redistribute
and re-license the relevant code from the fixest project under the MIT
License. The relevant code is the following:

• The script to generate benchmark data (https://github.com/lrberge/fixest/blob/master/_BENCHMARK/Data%20generation.R)
• The c++ function cpp_cholesky (https://github.com/lrberge/fixest/blob/dbe56c5d953deb797a61dfc37f6e25fd143c68d4/src/lm_related.cpp#L105).

Thank you! =)