Highlight partner wall on link-toggle hover

Hovering a wall-junction link-toggle icon today only swaps the icon
shape — the user doesn't see which wall the click will disconnect from
until after they click. ATPATH (T-junction) configurations especially
make the partner ambiguous when multiple connections sit close together.

On hover, paint a wireframe bbox around the partner wall using the same
shader, constants and color the array module already established for
its layer-children highlight (POLYLINE_UNIFORM_COLOR, decorator_color_special,
line width 1.8, alpha 0.8). The line-width / alpha constants in decorator.py
are renamed from _ARRAY_LAYER_BBOX_LINE_* to _BBOX_HIGHLIGHT_LINE_* and
shared between draw_array_layer_children_bbox and the new
draw_wall_partner_bbox so the two highlights stay in lockstep.

The trigger lives in a new GizmoWallLinkToggle subclass in wall.py
which keeps the base gizmos.GizmoLinkToggle generic (per the
generic-naming convention for shared widgets). The subclass's draw()
calls super().draw(context) then on self.is_highlight outlines its
partner_obj via the shared decorator helper. Same trigger pattern as
GizmoArrayLayerIndicator.

Blender's Gizmo API exposes target_set_operator but no symmetric
getter, so the partner reference can't be read back from the bound
operator handle. Instead GizmoWallUnjoinSingle.position_gizmos
mirrors the resolved partner_obj onto each visible icon every frame
next to the existing other_wall_guid write — the icon's draw() reads
from its own __slots__-declared attribute.

A forward-compat AST test pins the contract: GizmoWallLinkToggle.draw
must reference is_highlight and call draw_wall_partner_bbox. Catches
the regression where someone tidies the draw() override into super()
or replaces the shared helper with an ad-hoc draw call.

Generated with the assistance of an AI coding tool.

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

@@ -28,6 +28,7 @@ rule is."""
 
 import ast
 import inspect
+import textwrap
 
 import pytest
 
@@ -59,3 +60,33 @@ def test_iter_path_connections_uses_path_connectable_predicate():
         "that strict predicate drops fillet-corner walls. Use "
         "is_path_connectable_wall instead."
     )
+
+
+def test_gizmo_wall_link_toggle_invokes_partner_bbox_helper():
+    """The wall subclass must call draw_wall_partner_bbox when its hover
+    state is active. Without this contract the partner-wall highlight
+    silently regresses if someone "tidies" the draw() override away."""
+    from bonsai.bim.module.model import wall as wall_module
+
+    source = textwrap.dedent(inspect.getsource(wall_module.GizmoWallLinkToggle.draw))
+    tree = ast.parse(source)
+    attr_names = {node.attr for node in ast.walk(tree) if isinstance(node, ast.Attribute)}
+    call_names: set[str] = set()
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.Call):
+            continue
+        if isinstance(node.func, ast.Attribute):
+            call_names.add(node.func.attr)
+        elif isinstance(node.func, ast.Name):
+            call_names.add(node.func.id)
+
+    assert "is_highlight" in attr_names, (
+        "GizmoWallLinkToggle.draw must gate its highlight call on self.is_highlight — "
+        "without it the partner outline would draw every frame, not just on hover."
+    )
+    assert "draw_wall_partner_bbox" in call_names, (
+        "GizmoWallLinkToggle.draw must call draw_wall_partner_bbox to render the "
+        "partner outline. The shared composite in decorator.py is the canonical "
+        "trigger for this feature; replacing it with an ad-hoc draw call would "
+        "drift from the array-children bbox styling."
+    )