继续按 align-linuxcnc 约束：补螺纹转换错误源码覆盖，测试通过

test-linuxcnc-source-link.sh

@@ -857,6 +857,33 @@ for case in "${threading_check_error_cases[@]}"; do
     fi
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
 done
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc
+# convert_straight() and convert_threading_cycle() reject invalid
+# spindle state/selector, rotary-axis motion, and G76 I/J/K geometry
+# before issuing synchronized threading motion.
+threading_convert_error_cases=(
+    "linuxcnc_g33_stopped_spindle_error|Spindle not turning in G33"
+    "linuxcnc_g33_1_stopped_spindle_error|Spindle not turning in G33.1"
+    "linuxcnc_g33_spindle_dollar_out_of_range_error|Invalid spindle (\$) number in G33 move"
+    "linuxcnc_g33_1_spindle_dollar_out_of_range_error|Invalid spindle (\$) number in G33.1 move"
+    "linuxcnc_g76_spindle_dollar_out_of_range_error|Invalid D-number in G76 cycle"
+    "linuxcnc_g76_stopped_spindle_error|Chosen spindle (0) not turning in G76"
+    "linuxcnc_g76_rotary_axis_error|Cannot move rotary axes with G76"
+    "linuxcnc_g76_i_zero_error|In G76, I must not be 0"
+    "linuxcnc_g76_j_zero_error|In G76, J must be greater than 0"
+    "linuxcnc_g76_k_less_equal_j_error|In G76, K must be greater than J"
+)
+for case in "${threading_convert_error_cases[@]}"; do
+    fixture=${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 $fixture.ngc to fail" >&2
+        exit 1
+    fi
+    grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
+done
 # Source basis: LinuxCNC src/emc/rs274ngc/interp_read.cc
 # read_items() only consumes a leading N word before item dispatch;
 # read_one_item() then uses the default reader table from interp_array.cc