The recut in a6341981d applied its patch against the wrong directory
and restored the six shared files to the previous tree without the
body-native hunks. This commit adds exactly those: the `body-native`
feature and optional dependency, the pinned `sam3dbody` registry entry
and its test, the backend arms and the module declaration. Working
tree untouched.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
9ff44e87a6