Add IconSlot placeholders + stair xN tread label

Add a clickable "xN" badge to GizmoStairEdition's edit row, mirroring
the array's popup-input UX: click opens a number dialog (no more
shift+click-into-modal). Text-only — no 2x2 grid glyph.

Structural changes that enable this cleanly:

* IconSlot.placeholder=True: slots reserve an X position in the row
  without auto-creating a gizmo. Subclasses resolve the reserved X via
  _slot_x_positions()[name] to place their own dynamic gizmos. Drops
  the brittle "remember to add extra_gap_before" workaround that would
  silently rot on slot reorders.

* Array bug fix: the count badge collided with the "-" icon because
  the slot manager placed count_minus at the cycle position (X=0.87)
  where ICON_NUMBER_X also lives. Migrating the badge to a placeholder
  slot lets the manager allocate the X naturally and the "-" no longer
  overlaps. ICON_NUMBER_X constant removed.

* IntegerInputDialogMixin in parametric_lifecycle.py: extracts the
  popup-dialog plumbing shared between InputArrayCount and the new
  InputStairTreads. Subclasses declare an IntProperty + attr_name +
  props_getter; the mixin owns invoke/execute. _resolve_props helper
  factors the common obj/props/requires_editing prologue.

Tests: BIM_GT_count_label registration; IconSlot placeholder contract
(no gizmo_idname required; gizmo_attrs() returns empty); the stair
edit-row slot layout reserves the label position between tread_lock
and plus at one ICON_ARRAY_GAP each; visibility propagates from
props.is_editing.

Partly generated with the assistance of an AI coding tool.

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

@@ -133,3 +133,75 @@ def test_set_icon_gizmo_position_does_not_apply_object_rotation():
     for row_a, row_b in zip(stub.matrix_basis, expected):
         for va, vb in zip(row_a, row_b):
             assert abs(va - vb) < 1e-6
+
+
+def test_icon_slot_placeholder_skips_validation_and_returns_no_attrs():
+    """Placeholder slots reserve an X position without an auto-created gizmo:
+    construction must not require ``gizmo_idname`` / ``operator``, and
+    ``gizmo_attrs()`` must return an empty tuple so the base class's
+    setup/positioning loops naturally skip the slot."""
+    from bonsai.bim.module.drawing.gizmos import IconSlot
+
+    slot = IconSlot(name="my_label", placeholder=True)
+    assert slot.placeholder is True
+    assert slot.gizmo_attrs() == ()
+
+    with pytest.raises(TypeError, match="gizmo_idname"):
+        IconSlot(name="broken")
+
+
+def test_count_label_gizmo_is_registered():
+    """The shared text-only ``xN`` gizmo must register so the stair group's
+    ``gizmos.new("BIM_GT_count_label")`` resolves."""
+    from bonsai.bim.module.drawing.gizmos import GizmoCountLabel
+
+    assert GizmoCountLabel.bl_idname == "BIM_GT_count_label"
+    assert bpy.types.Gizmo.bl_rna_get_subclass_py("BIM_GT_count_label") is GizmoCountLabel
+
+
+def test_stair_edit_row_reserves_label_slot_between_tread_lock_and_plus():
+    """The ``tread_count_label`` placeholder slot must sit one
+    ``ICON_ARRAY_GAP`` past the tread-lock and one gap before the plus
+    icon, so the layout naturally allocates the count label's X without
+    any subclass-side gap math."""
+    from bonsai.bim.module.drawing.gizmos import BaseParametricGizmoGroup
+    from bonsai.bim.module.model.stair import GizmoStairEdition
+
+    slot_x = GizmoStairEdition._slot_x_positions()
+    gap = BaseParametricGizmoGroup.ICON_ARRAY_GAP
+
+    assert "tread_count_label" in slot_x
+    assert slot_x["tread_count_label"] - slot_x["tread_lock"] == pytest.approx(gap)
+    assert slot_x["plus"] - slot_x["tread_count_label"] == pytest.approx(gap)
+    assert slot_x["minus"] - slot_x["plus"] == pytest.approx(gap)
+
+
+def test_update_tread_count_gizmos_toggles_label_with_editing():
+    """``update_tread_count_gizmos`` must propagate ``props.is_editing``
+    to the label's hide state so the badge appears only inside edit mode."""
+    from bonsai.bim.module.drawing.gizmos import BaseParametricGizmoGroup
+    from bonsai.bim.module.model.stair import GizmoStairEdition
+
+    class _GizmoStub:
+        def __init__(self):
+            self.hide = False
+
+    plus_gz = _GizmoStub()
+    minus_gz = _GizmoStub()
+    label_gz = _GizmoStub()
+
+    fake_self = types.SimpleNamespace(
+        plus_gizmo=plus_gz,
+        minus_gizmo=minus_gz,
+        tread_count_label_gizmo=label_gz,
+        update_gizmo_visibility=lambda g, v: BaseParametricGizmoGroup.update_gizmo_visibility(fake_self, g, v),
+        is_gizmo_hidden_by_modal=lambda g: False,
+    )
+
+    props_editing = types.SimpleNamespace(is_editing=True, number_of_treads=5)
+    GizmoStairEdition.update_tread_count_gizmos(fake_self, props_editing)
+    assert label_gz.hide is False
+
+    props_idle = types.SimpleNamespace(is_editing=False, number_of_treads=5)
+    GizmoStairEdition.update_tread_count_gizmos(fake_self, props_idle)
+    assert label_gz.hide is True