baton build hardcodes python3, which breaks the plugin build on stock Windows

Describe the bug
The baton custom_target invokes the literal command python3 (xplane_plugin/subprojects/baton/meson.build:19-30). On a stock Windows install the interpreter is python.exe or the py launcher, and python3 does not resolve.

That target is the sole producer of libbaton.a, lib.rs.cc and lib.rs.h, so the failure blocks the entire plugin build.

To Reproduce

1. On Windows without python3 on PATH, run meson setup build then meson compile -C build in xplane_plugin/.
2. The baton target fails.

Expected behavior
The build finds whichever Python 3 interpreter is present.

Additional context
Meson has import('python').find_installation() for exactly this. The failure is also hard to diagnose: it surfaces as a FileNotFoundError from shutil.copyfile in script.py rather than as a missing interpreter.

Found while writing the setup documentation in #138 / #178.