Add cursor-aware extend-arrow flip on wall edit gizmos

The extend-X / extend-Z icons in GizmoWallEdition's cursor row are
billboarded toward the camera; without orientation polish they
always point in the same screen-space direction regardless of which
wall endpoint the click will move (or whether the cursor sits above
or below the wall top). New helper mirrors the icon's local-X (extend-X)
or local-Y (extend-Z) axis so each arrow points toward the end it
will move:

* Extend-X: walk wall midpoint to figure out which endpoint stays
  fixed (cursor past midpoint → ATSTART stays; cursor before midpoint
  → ATEND stays). Project the fixed endpoint into screen-space and
  flip the arrow when the gizmo's anchor sits on the same side.
* Extend-Z: flip when the cursor is below the wall top (within
  EXTEND_FLIP_EPSILON tolerance).

Called once per resolved cursor gizmo from
``GizmoWallEdition._update_cursor_gizmos``, after the gizmo's
``matrix_basis`` is set by ``gizmo.billboarded_at``. Reuses
``gizmo.should_flip_extend_arrow`` + ``EXTEND_FLIP_MIRROR_X/Y`` +
``EXTEND_FLIP_EPSILON`` already on tool.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/model/wall.py

@@ -2133,6 +2133,7 @@ class GizmoWallEdition(bpy.types.GizmoGroup, gizmo.BaseParametricGizmoGroup):
             gz.hide = self.is_gizmo_hidden_by_modal(gz)
             world_pos = mw @ Vector((cursor_local.x, 0.0, local_z))
             gz.matrix_basis = gizmo.billboarded_at(world_pos, billboard_rot)
+            _apply_wall_extend_flips(gz, self, world_pos, mw, cursor_local, props, billboard_rot)
 
     def _update_icon_row_extras(self, context: bpy.types.Context, mw: Matrix, props: "BIMWallProperties") -> None:
         """Position the wall-specific icons in the icon row.
@@ -2195,6 +2196,32 @@ class GizmoWallEdition(bpy.types.GizmoGroup, gizmo.BaseParametricGizmoGroup):
             self.toggle_openings_gizmo.hide = True
 
 
+def _apply_wall_extend_flips(
+    gz: bpy.types.Gizmo,
+    group: "GizmoWallEdition",
+    world_pos: Vector,
+    mw: Matrix,
+    cursor_local: Vector,
+    props: "BIMWallProperties",
+    billboard_rot: Matrix,
+) -> None:
+    """Mirror the wall's extend arrows so each points toward the end the click will move.
+
+    Extend-X: arrow points away from the wall endpoint that the operator would
+    keep fixed, accounting for the camera's screen-X orientation. Extend-Z:
+    arrow flips downward when the cursor sits below the wall top."""
+    if gz is group.extend_x_gizmo:
+        if props.length > 0 and cursor_local.x > props.anchor_x + props.length / 2:
+            reference_x = props.anchor_x
+        else:
+            reference_x = props.anchor_x + props.length
+        reference_world = mw @ Vector((reference_x, 0.0, 0.0))
+        if gizmo.should_flip_extend_arrow(world_pos, reference_world, billboard_rot):
+            gz.matrix_basis = gz.matrix_basis @ gizmo.EXTEND_FLIP_MIRROR_X
+    elif gz is group.extend_z_gizmo and cursor_local.z < props.height - gizmo.EXTEND_FLIP_EPSILON:
+        gz.matrix_basis = gz.matrix_basis @ gizmo.EXTEND_FLIP_MIRROR_Y
+
+
 def _commit_active_wall_edit_if_any(context: bpy.types.Context) -> bpy.types.Object | None:
     """Return the active object, committing any in-progress wall edit first.