继续按 align-linuxcnc 约束：补 G7G8 源链接回归

结论：新增 LinuxCNC-backed lathe diameter/radius fixture，依据 interp_convert.cc::convert_lathe_diameter_mode 与 interp_namedparams.cc readonly 参数，在 native librs274 与 source-link runner 中验证 G7/G8 同块 X 缩放和 #<_lathe_diameter_mode>/#<_lathe_radius_mode> 分支；未扩展 smoke，test-native 与 test-linuxcnc-source-link 均通过。

test-all-native.sh

@@ -556,6 +556,8 @@ if grep -F 'read -r -a python_includes <<<"$(' list-linuxcnc-source-common-cxxfl
     echo "source common cxxflags still hides python-config failures behind read" >&2
     exit 1
 fi
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-native.sh >/dev/null
+grep -F 'tests/gcode/linuxcnc_lathe_diameter_mode.ngc' test-native.sh >/dev/null
 
 for script in \
     test-native.sh \