按建议，继续工作

结论：已新增 G20/G21 单位切换 fixture，固定 vendored LinuxCNC convert_length_units() 产生的 USE_LENGTH_UNITS canonical 边界；native probe 全量验证通过。

wasm-port/docs/porting-steps-standalone.md

@@ -380,6 +380,10 @@ Current verified progress:
   `interp_convert.cc::convert_coordinate_system()`,
   `interp_convert.cc::convert_setup()`, and
   `interp_convert.cc::convert_axis_offsets()`.
+- `tests/fixtures/gcode/length_units.ngc` and
+  `tests/fixtures/canon/length_units.events` pin `G20`/`G21` unit switching
+  and the `USE_LENGTH_UNITS` canonical boundary emitted by vendored
+  `interp_convert.cc::convert_length_units()`.
 - `runtime/core/linuxcnc_wrap/linuxcnc_interp_minimal_runtime.cpp` now records
   feed/control canonical boundary calls emitted by vendored LinuxCNC
   conversion and queue code: `SET_TRAVERSE_RATE`, `SET_FEED_REFERENCE`,

wasm-port/docs/scope-and-baseline.md

@@ -35,6 +35,7 @@ G-code fixture coverage includes:
 - linear traverse and feed
 - arc semantics
 - modal incremental and absolute distance modes
+- length unit switching
 - position parameters
 - canned cycles
 - coordinate offsets

wasm-port/tests/fixtures/canon/length_units.events

@@ -0,0 +1,8 @@
+canon_event=SELECT_PLANE plane=1
+canon_event=STRAIGHT_TRAVERSE line=1 x=25.4 y=0 z=0 a=0 b=0 c=0 u=0 v=0 w=0
+canon_event=USE_LENGTH_UNITS units=1
+canon_event=SET_FEED_RATE rate=60
+canon_event=STRAIGHT_FEED line=3 x=1.5 y=0 z=0 a=0 b=0 c=0 u=0 v=0 w=0
+canon_event=USE_LENGTH_UNITS units=2
+canon_event=SET_FEED_RATE rate=120
+canon_event=STRAIGHT_FEED line=5 x=50.8 y=0 z=0 a=0 b=0 c=0 u=0 v=0 w=0

wasm-port/tests/fixtures/gcode/length_units.ngc

@@ -0,0 +1,5 @@
+G90 G17 G0 X25.4 Y0 Z0
+G20
+G1 X1.5 F60
+G21
+G1 X50.8 F120