继续按 align-linuxcnc 约束：补只读命名参数源回归

结论：新增 LinuxCNC-backed readonly named parameter fixture，依据 interp_namedparams.cc::lookup_named_param 与 init_readonly_param，在 native librs274 与 source-link runner 中验证 #<_metric>/#<_imperial>/#<_absolute>/#<_incremental>/#<_feed>/#<_rpm>/#<_spindle_on>/#<_spindle_cw>；未扩展 smoke，test-native 与 test-linuxcnc-source-link 均通过。

test-all-native.sh

@@ -559,6 +559,7 @@ fi
 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
+grep -F 'tests/gcode/linuxcnc_readonly_named_parameters.ngc' test-native.sh >/dev/null
 
 for script in \
     test-native.sh \