继续按 align-linuxcnc 约束：补 G/M reader 源码错误覆盖

结论：覆盖 LinuxCNC interp_read.cc/read_g()/read_m() 的范围、未知代码和同 modal group 冲突错误，代码表依据 interp_array.cc；未扩展 smoke。

验证：./test-linuxcnc-source-link.sh；./test-native.sh。

test-linuxcnc-source-link.sh

@@ -710,6 +710,28 @@ for case in "${negative_word_error_cases[@]}"; do
     grep -q "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
 done
 # Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc
+# read_g()/read_m() validate numeric range, known code tables from
+# interp_array.cc, and same-modal-group conflicts while filling a block.
+gm_reader_error_cases=(
+    "linuxcnc_g_code_out_of_range_error|G-code out of range"
+    "linuxcnc_unknown_g_code_error|Unknown g code used"
+    "linuxcnc_two_g_codes_same_modal_group_error|Two g codes used from same modal group"
+    "linuxcnc_m_code_greater_199_error|M-code greater than 199: M200"
+    "linuxcnc_unknown_m_code_error|Unknown m code used: M13"
+    "linuxcnc_two_m_codes_same_modal_group_error|Two m codes used from same modal group"
+)
+for case in "${gm_reader_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" \
+        "$build_dir/linuxcnc_rs274_source_dump" "tests/gcode/$fixture.ngc" \
+        >"$output_dir/cnc_sim_linuxcnc_source_$fixture.json" 2>"$output_dir/cnc_sim_linuxcnc_source_$fixture.err"; then
+        echo "expected $fixture.ngc to fail" >&2
+        exit 1
+    fi
+    grep -q "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
+done
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc
 # the word-specific reader functions reject repeated words while
 # filling a single block.
 multiple_word_error_cases=(