Add cursor-bound perpendicular wall gizmo

GizmoWallEdition gains a fourth cursor-anchored icon that
spawns a perpendicular branch wall from the cursor's
orthogonal projection on the source wall axis. Click forms
a T-junction; shift+click forms an L-corner with the source
wall trimmed at the projection, keeping its longer portion.

The branch inherits the source's spatial container and
centerline baseline so its authored axis matches the source's
alignment rather than the type's default.

Also includes a floor-plane preview quad for the new gizmo,
a floor-Z cross line on the split preview for top-down
visibility, a small bump to QUAD_ALPHA for clearer preview
fills, and a stacking-offset helper that centralises the
cursor-row screen-up step across three call sites.

Generated with the assistance of an AI coding tool.

src/bonsai/test/bim/module/model/test_wall_gizmos.py

@@ -302,3 +302,92 @@ def test_iter_path_connections_walks_both_inverses_in_order():
     rel_from = _make_path_rel(relating=p2, related=self_elem, relating_ct="ATEND", related_ct="ATEND")
     elem = SimpleNamespace(ConnectedTo=[rel_to], ConnectedFrom=[rel_from])
     assert _run_iter_path_connections(elem) == [(p1, "ATSTART", "ATSTART"), (p2, "ATEND", "ATEND")]
+
+
+# ----------------------------------------------------------------------------
+# _perpendicular_wall_params — clamping + side detection for the
+# "add perpendicular wall at cursor" gizmo and its operator.
+# ----------------------------------------------------------------------------
+#
+# Pure scalar math. The dead-zone is ``CURSOR_STACK_OFFSET`` — inside it the
+# on-axis split / extend-X icons own the click and this helper returns None.
+
+
+def _wall_consts():
+    from bonsai.bim.module.model.wall import GizmoWallEdition
+
+    return GizmoWallEdition.CURSOR_STACK_OFFSET
+
+
+def _run_perp_params(cursor_x, cursor_y, anchor_x=0.0, length=5.0):
+    from bonsai.bim.module.model.wall import _perpendicular_wall_params
+
+    return _perpendicular_wall_params(cursor_x, cursor_y, anchor_x, length)
+
+
+def test_perpendicular_params_on_axis_returns_none():
+    assert _run_perp_params(cursor_x=2.0, cursor_y=0.0) is None
+
+
+def test_perpendicular_params_at_dead_zone_boundary_returns_none():
+    # Inclusive boundary: at exactly the threshold the on-axis icons still own
+    # the click; the gizmo only takes over strictly past the dead zone.
+    threshold = _wall_consts()
+    assert _run_perp_params(cursor_x=2.0, cursor_y=threshold) is None
+    assert _run_perp_params(cursor_x=2.0, cursor_y=-threshold) is None
+
+
+def test_perpendicular_params_just_past_dead_zone_returns_params():
+    threshold = _wall_consts()
+    result = _run_perp_params(cursor_x=2.0, cursor_y=threshold + 0.01)
+    assert result is not None
+    clamped_x, length, side = result
+    assert clamped_x == pytest.approx(2.0)
+    assert length == pytest.approx(threshold + 0.01)
+    assert side == 1.0
+
+
+def test_perpendicular_params_negative_y_flips_side_sign():
+    result = _run_perp_params(cursor_x=2.0, cursor_y=-1.5)
+    assert result is not None
+    _, length, side = result
+    # Length is always positive — the side sign carries the direction so the
+    # operator can pick the +90° vs -90° rotation without sign-flipping length.
+    assert length == pytest.approx(1.5)
+    assert side == -1.0
+
+
+def test_perpendicular_params_clamps_low_when_cursor_left_of_wall():
+    result = _run_perp_params(cursor_x=-2.0, cursor_y=1.5, anchor_x=0.0, length=5.0)
+    assert result is not None
+    clamped_x, _length, _side = result
+    assert clamped_x == pytest.approx(0.0)
+
+
+def test_perpendicular_params_clamps_high_when_cursor_right_of_wall():
+    result = _run_perp_params(cursor_x=10.0, cursor_y=1.5, anchor_x=0.0, length=5.0)
+    assert result is not None
+    clamped_x, _length, _side = result
+    assert clamped_x == pytest.approx(5.0)
+
+
+def test_perpendicular_params_respects_nonzero_anchor_x():
+    # Non-zero anchor_x shifts the wall span; clamping must follow.
+    result = _run_perp_params(cursor_x=0.5, cursor_y=1.5, anchor_x=2.0, length=5.0)
+    assert result is not None
+    clamped_x, _length, _side = result
+    assert clamped_x == pytest.approx(2.0)
+
+    result = _run_perp_params(cursor_x=10.0, cursor_y=1.5, anchor_x=2.0, length=5.0)
+    assert result is not None
+    clamped_x, _length, _side = result
+    assert clamped_x == pytest.approx(7.0)
+
+
+def test_perpendicular_params_in_range_passes_cursor_x_through():
+    result = _run_perp_params(cursor_x=3.0, cursor_y=1.5, anchor_x=0.0, length=5.0)
+    assert result is not None
+    clamped_x, length, side = result
+    assert clamped_x == pytest.approx(3.0)
+    assert length == pytest.approx(1.5)
+    assert side == 1.0