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-native.sh

@@ -140,6 +140,9 @@ grep -F 'expected LinuxCNC INI PARAMETER_FILE to restore numbered parameters' co
 grep -F 'expected LinuxCNC config INI_FILE_NAME to back #<_ini[]> named parameters' core/tests/cnc_sim_api_linuxcnc_rs274_smoke.cpp >/dev/null
 grep -F 'interp->ini_load(options.ini_file_name.c_str())' core/src/linuxcnc_rs274_backend.cpp >/dev/null
 grep -F 'ScopedEnvironmentVariable ini_file_env("INI_FILE_NAME", options.ini_file_name)' core/src/linuxcnc_rs274_backend.cpp >/dev/null
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-linuxcnc-rs274-native.sh >/dev/null
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'tests/gcode/linuxcnc_lathe_diameter_mode.ngc' docs/linuxcnc-porting.md >/dev/null
 grep -F 'rs274ngc_pre.cc ini_load() consumes INI_FILE_NAME' test-web-wasm-node-smoke.cjs >/dev/null
 grep -F 'rs274ngc_pre.cc ini_load() consumes INI_FILE_NAME' web/test-browser-wasm-smoke-linuxcnc-sections.js >/dev/null
 grep -F 'metadata:src/emc/nml_intf/emcops.cc:source basis for native source-link EMC status constructor support object' linuxcnc-rs274-source-files.txt >/dev/null