- Remove AssemblerMeta.resolve_params in favor of Step.get_assembler_kwargs()
- Replace step.assembler_name/assembler_params with ASSEMBLER_NAME ClassVar
  and real typed attributes on each Step subclass
- Add MachineDefaults dataclass wired through the pipeline
- Thread Step through pipeline instead of separate assembler_name/params
- Add AssemblerRegistry with _register_laser_assemblers fallback for workers
- Make task_mgr a required parameter on AddonManager, passed at construction
  so _build_and_update_manifest always finds shared_state
- Remove set_task_manager method (redundant with constructor parameter)
- Remove _ensure_assemblers_registered conftest fixture (assembler registry
  fallback handles it)
- Step.from_dict dispatches via step_registry; workers get addon manifest
  through shared_state automatically
- Remove _STEP_TYPE_TO_ASSEMBLER hack from compute_workpiece_artifact
- Add to_dict/from_dict roundtrip and get_assembler_kwargs tests for all
  step subclasses
- Clean up pipeline test args indices (args[7] -> args[5])
- Remove addon imports from core pipeline tests

- Add orchestration ClassVars to Step base (IS_VECTOR, SPLIT_CONTOURS,
  SET_POWER, NORMALIZE_WINDINGS, ALWAYS_WRAP, SECTION_TYPE) and
  override on each Step subclass - replaces AssemblerMeta fields
- Delete AssemblerMeta class entirely; AssemblerRegistry becomes a
  plain name -> Callable map with no param_keys filtering
- Add set_step_property() helper to StepComponentSettingsWidget base
  for undoable step attribute writes via ChangePropertyCommand
- Migrate contour, frame, shrinkwrap, wavefront, and raster widgets
  to read/write step attributes directly instead of opsproducer_dict
- Add override_threshold, threshold to ContourStep and dither_algorithm
  to EngraveStep (previously only on producers)
- Fix Step.from_dict to delegate to step_class.from_dict(data) so
  subclass attributes are deserialized correctly
- Remove _register_laser_assemblers() hardcoded fallback from registry
  (consistent with ProducerRegistry/StepRegistry pattern)
- Simplify all assembler registration calls (no AssemblerMeta wrapper)
- ShrinkWrapStep imports prepare_surface directly from rayforge.image
- Fix UnboundLocalError from local logger shadowing module-level logger

Re-implement bidirectional raster scan X-offset calibration feature
from main in the new assembler/step architecture (producers were
deleted in this branch):

- EngraveStep: add bidir_x_offset_mm attribute with serialization
- MaterialTestStep: add offset_range attribute, pass min/max_offset to
  assembler, add SPEED_VS_OFFSET grid mode support
- material_test_helpers: add SPEED_VS_OFFSET to GridMode enum, pass
  min/max_offset to draw_preview
- material_test_cmd: add offset_range to _PARAM_KEYS for preview sync
- raster_widget: read bidir_x_offset_mm from self.step (not producer)
- material_test_grid_widget: read offset_range from self.step, fix
  offset handlers to use tuple signature of _update_range_param
- workpiece_stage: read bidir_x_offset_mm from step.to_dict() via
  setdefault instead of deleted opsproducer_dict
- Add BidirScanOffsetTransformer and tests from main (unchanged)
- Update tests for new architecture (no opsproducer_dict references)