Add libffi 3.8.0 and Python 3.14.10 to debian13

Python is built from the python.org tarball against the pinned OpenSSL,
zlib, Zstd, SQLite and libffi; it does not pass --libdir because the
standard library and lib-dynload must stay under lib/python3.14. SQLite
is static, so its link flags are taken from pkg-config --static.
libffi is built with --disable-multi-os-directory so it installs into
the multiarch directory instead of lib/lib.

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

fetch-sources.sh

@@ -168,3 +168,7 @@ fetch wireguard-tools-v1.0.20260223.tar.xz \
     https://git.zx2c4.com/wireguard-tools/snapshot/wireguard-tools-v1.0.20260223.tar.xz
 fetch freetds-1.5.21.tar.gz \
     https://www.freetds.org/files/stable/freetds-1.5.21.tar.gz
+fetch libffi-3.8.0.tar.gz \
+    https://github.com/libffi/libffi/releases/download/v3.8.0/libffi-3.8.0.tar.gz
+fetch Python-3.14.10.tgz \
+    https://www.python.org/ftp/python/3.14.10/Python-3.14.10.tgz