按建议，继续工作

结论：已新增 G17/G18/G19 平面选择 fixture，固定 vendored LinuxCNC convert_set_plane() 产生的 SELECT_PLANE canonical 边界；native probe 全量验证通过。

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

@@ -384,6 +384,10 @@ Current verified progress:
   `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()`.
+- `tests/fixtures/gcode/plane_selection.ngc` and
+  `tests/fixtures/canon/plane_selection.events` pin `G17`/`G18`/`G19` plane
+  switching and the `SELECT_PLANE` canonical boundary emitted by vendored
+  `interp_convert.cc::convert_set_plane()`.
 - `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

@@ -36,6 +36,7 @@ G-code fixture coverage includes:
 - arc semantics
 - modal incremental and absolute distance modes
 - length unit switching
+- plane selection
 - position parameters
 - canned cycles
 - coordinate offsets

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

@@ -0,0 +1,3 @@
+canon_event=SELECT_PLANE plane=3
+canon_event=SELECT_PLANE plane=2
+canon_event=SELECT_PLANE plane=1

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

@@ -0,0 +1,3 @@
+G18
+G19
+G17