Desktop jar: ship a stripped linux libpython (252 MB unstripped today) #86

After #74 the linux libpython3.14.so in the desktop jar is the real file but 251,884,016 bytes: python-build-standalone's install_only build carries debug info, and the best-effort strip finds no ELF-capable strip on a macOS build host (no ELF-capable strip found; libpython3.14.so ships unstripped). macOS dylibs are ~19 MB.

Fix: acquire PBS's install_only_stripped archives (pin URL + SHA-256 like the others in python-checksums.properties) for linux (and check windows/macos variants), instead of relying on a host strip tool. Measure the jar before/after.

Completion: the jar's linux libpython is a stripped ELF (size recorded), checksums pinned, a build check fails if a linux libpython in the jar has a .debug_info section.