继续按 align-linuxcnc 约束：补 G7G76 源链接回归

结论：新增 LinuxCNC-backed G7 G76 fixture，依据 interp_convert.cc::convert_lathe_diameter_mode 和 convert_threading_cycle 验证直径模式下 G76 I/J/K 几何缩放、最终 threading pass 与 taper sync pitch；未扩展 smoke，test-native 与 test-linuxcnc-source-link 均通过。

test-all-native.sh

@@ -556,8 +556,9 @@ 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 'convert_lathe_diameter_mode() scales G7/G8 X words and G76 I/J/K words' test-native.sh >/dev/null
 grep -F 'tests/gcode/linuxcnc_lathe_diameter_mode.ngc' test-native.sh >/dev/null
+grep -F 'tests/gcode/linuxcnc_lathe_diameter_g76.ngc' test-native.sh >/dev/null
 
 for script in \
     test-native.sh \