Fix in-place alignment editing and curve visualization

- Add clear_layout_segments API to remove segments while preserving alignment ID
- Modify exit_pi_edit_mode to edit segments in-place instead of delete+recreate
- Fix curve visualization by using create_shape for segment vertices
- Fix evaluate_segment validation to handle negative-length curve segments

This prevents "Active alignment no longer exists" errors when editing PIs
and properly renders circular arcs regardless of turn direction.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ifcopenshell-python/ifcopenshell/api/alignment/__init__.py

@@ -51,6 +51,7 @@ from ._get_segment_start_point_label import register_referent_name_callback
 from .add_stationing_referent import add_stationing_referent
 from .add_vertical_layout import add_vertical_layout
 from .add_zero_length_segment import add_zero_length_segment
+from .clear_layout_segments import clear_layout_segments
 from .create import create
 from .create_as_offset_curve import create_as_offset_curve
 from .create_as_polyline import create_as_polyline
@@ -93,6 +94,7 @@ __all__ = [
     "add_stationing_referent",
     "add_vertical_layout",
     "add_zero_length_segment",
+    "clear_layout_segments",
     "create",
     "create_as_offset_curve",
     "create_as_polyline",