Add wall parametric editing and gizmos

Walls gain in-viewport parametric editing matching the door/window/stair
UX: drag handles for length, height, slope (x-angle), layer baseline
cycle, plus cursor-anchored quality-of-life operators (split at cursor,
extend to cursor, extend height, rotate 90, toggle openings) and
two-object state-machine gizmos (unjoin / merge / join-corner /
extend-to-wall / extend-vertically / add-opening).

Wall enters tool.Parametric.EDIT_TYPES, so save-time auto-commit,
GizmoPreferencesWall registration, and the in-progress-edit predicates
all light up automatically through the registry plumbing landed two
commits back.

The three-layer commit model (drag -> BIMWallProperties -> bmesh
preview -> Finish -> single ifc.run) means dragging a handle through
hundreds of intermediate values produces zero extra IFC entities. A
no-op enable->finish round-trip is byte-identical. The snapshot diff
in FinishEditingWall skips unchanged params.
_commit_active_wall_edit_if_any ensures cursor-anchored operators see
committed geometry, not the draft preview box.

Also lands the `prompt_auto_commit_parametric_edits` BoolProperty on
BIM_ADDON_preferences (consumed by the auto-commit dialog landed in
the framework commit) and refactors
`draw_{door,window,stair}_gizmo_parameters` into a shared
`_draw_parametric_gizmo_parameters` helper that the new
`draw_wall_gizmo_parameters` reuses. This commit and the framework
commit are stacked - the framework commit references the BoolProperty
defined here, so they must land together.

Tests cover pure math (core/test_model.py), DimensionGizmoConfig text
formatter, GizmoWallExtendVertically.poll() preconditions, and the
refresh_post_commit cache-invalidation regression. BDD scenarios in
model.feature cover the edit triad, auto-commit on save, and the
two-object gizmos. Documentation added to creating_walls.rst.

Generated with the assistance of an AI coding tool.

src/bonsai/test/bim/test_feature.py

@@ -15,6 +15,8 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was modified with the assistance of an AI coding tool.
 
 from __future__ import annotations
 
@@ -1131,6 +1133,17 @@ def the_variable_key_is_value(key, value):
     variables[key] = eval(replace_variables(value))
 
 
+@then(parsers.parse('the variable "{key}" equals "{value}"'))
+def the_variable_key_equals_value(key, value):
+    assert key in variables, f'Variable "{key}" was never set'
+    expected = eval(replace_variables(value))
+    actual = variables[key]
+    if isinstance(actual, float) and isinstance(expected, float):
+        assert abs(actual - expected) < 1e-5, f'Variable "{key}" is {actual!r}, expected {expected!r}'
+    else:
+        assert actual == expected, f'Variable "{key}" is {actual!r}, expected {expected!r}'
+
+
 @then("nothing happens")
 def nothing_happens():
     pass