Bonsai: add viewport decorator base helpers

Add two helpers to tool.Blender that 17+ existing viewport decorators
re-implement byte-identically:

- ViewportDecorator.draw_batch(shader_type, content_pos, color, indices=None)
  collapses the validate + batch_for_shader + uniform_float + draw cycle
  every shader-driven decorator needs.
- Blender.transparent_color(color, alpha=0.1) is the RGBA-alpha-override
  helper duplicated across nest, project, aggregate, model, system module
  scopes plus six nested-def copies inside draw methods.

Pure additions on the tool/ layer with direct unit tests covering the
default-alpha, explicit-alpha, non-mutation, new-list-instance, and
validation-guard branches.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/blender.py

@@ -538,6 +538,19 @@ class Blender(bonsai.core.tool.Blender):
             cls.handlers.clear()
             cls.is_installed = False
 
+        def draw_batch(self, shader_type, content_pos, color, indices=None):
+            """Submit a GPU batch through ``self.line_shader`` (for ``"LINES"``)
+            or ``self.shader`` (for any other primitive). Skips empty batches
+            via ``validate_shader_batch_data`` so Blender 4.4+ doesn't crash on
+            empty ``indices``. Subclasses bind both shaders in their draw method
+            before calling this helper."""
+            if not Blender.validate_shader_batch_data(content_pos, indices):
+                return
+            shader = self.line_shader if shader_type == "LINES" else self.shader
+            batch = batch_for_shader(shader, shader_type, {"pos": content_pos}, indices=indices)
+            shader.uniform_float("color", color)
+            batch.draw(shader)
+
         @staticmethod
         def _lookup_active_instance(gizmo_cls: type, context: bpy.types.Context) -> Optional[Any]:
             """Return the live ``GizmoGroup`` instance registered under
@@ -2391,6 +2404,13 @@ class Blender(bonsai.core.tool.Blender):
             return False
         return True
 
+    @staticmethod
+    def transparent_color(color: Iterable[float], alpha: float = 0.1) -> list[float]:
+        """Copy an RGBA color with its alpha channel overridden."""
+        out = [c for c in color]
+        out[3] = alpha
+        return out
+
     @classmethod
     def draw_bmesh_face_tris(
         cls,

src/bonsai/test/tool/test_blender.py

@@ -42,6 +42,46 @@ class TestImplementsTool(NewFile):
         assert isinstance(subject(), bonsai.core.tool.Blender)
 
 
+class TestTransparentColor(NewFile):
+    def test_default_alpha_overrides_to_zero_one(self):
+        assert subject.transparent_color([1.0, 0.5, 0.25, 1.0]) == [1.0, 0.5, 0.25, 0.1]
+
+    def test_explicit_alpha_is_applied(self):
+        assert subject.transparent_color([1.0, 0.5, 0.25, 1.0], alpha=0.5) == [1.0, 0.5, 0.25, 0.5]
+
+    def test_does_not_mutate_input(self):
+        original = [1.0, 0.5, 0.25, 1.0]
+        subject.transparent_color(original)
+        assert original == [1.0, 0.5, 0.25, 1.0]
+
+    def test_returns_new_list_instance(self):
+        original = [1.0, 0.5, 0.25, 1.0]
+        result = subject.transparent_color(original)
+        assert result is not original
+
+
+class TestViewportDecoratorDrawBatch(NewFile):
+    def test_empty_content_pos_skips_shader_calls(self):
+        from unittest.mock import MagicMock
+
+        decorator = subject.ViewportDecorator()
+        decorator.line_shader = MagicMock()
+        decorator.shader = MagicMock()
+        decorator.draw_batch("LINES", [], (1.0, 1.0, 1.0, 1.0))
+        decorator.line_shader.uniform_float.assert_not_called()
+        decorator.shader.uniform_float.assert_not_called()
+
+    def test_empty_indices_skips_shader_calls(self):
+        from unittest.mock import MagicMock
+
+        decorator = subject.ViewportDecorator()
+        decorator.line_shader = MagicMock()
+        decorator.shader = MagicMock()
+        decorator.draw_batch("LINES", [(0.0, 0.0, 0.0), (1.0, 0.0, 0.0)], (1.0, 1.0, 1.0, 1.0), indices=[])
+        decorator.line_shader.uniform_float.assert_not_called()
+        decorator.shader.uniform_float.assert_not_called()
+
+
 class TestCopyNodeGraph(NewFile):
     def test_run(self):
         material_to = bpy.data.materials.new("material_to")