Add missing markDirty()/markCompositeDirty() to all Python property setters

Fixes a systemic bug where Python tp_getset property setters bypassed the
render cache dirty flag system (#144). The animation/C++ setProperty() path
had correct dirty propagation, but direct Python property assignments
(e.g. frame.x = 50, caption.text = "Hello") did not invalidate the parent
Frame's render cache when clip_children or cache_subtree was enabled.

Changes by file:
- UIDrawable.cpp: Add markCompositeDirty() to set_float_member (x/y),
  set_pos, set_grid_pos; add markDirty() for w/h resize
- UICaption.cpp: Add markDirty() to set_text, set_color_member,
  set_float_member (outline/font_size); markCompositeDirty() for position
- UICollection.cpp: Add markContentDirty() on owner in append, remove,
  pop, insert, extend, setitem, and slice assignment/deletion
- UISprite.cpp: Add markDirty() to scale/sprite_index/texture setters;
  markCompositeDirty() to position setters
- UICircle.cpp: Add markDirty() to radius/fill_color/outline_color/outline;
  markCompositeDirty() to center setter
- UILine.cpp: Add markDirty() to start/end/color/thickness setters
- UIArc.cpp: Add markDirty() to radius/angles/color/thickness setters;
  markCompositeDirty() to center setter
- UIGrid.cpp: Add markDirty() to center/zoom/camera_rotation/fill_color/
  size/perspective/fov setters

Closes #288, closes #289, closes #290, closes #291

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/UILine.cpp

@@ -423,6 +423,7 @@ int UILine::set_start(PyUILineObject* self, PyObject* value, void* closure) {
         return -1;
     }
     self->data->setStart(vec->data);
+    self->data->markDirty(); // #291: visual change
     return 0;
 }
 
@@ -443,6 +444,7 @@ int UILine::set_end(PyUILineObject* self, PyObject* value, void* closure) {
         return -1;
     }
     self->data->setEnd(vec->data);
+    self->data->markDirty(); // #291: visual change
     return 0;
 }
 
@@ -462,6 +464,7 @@ int UILine::set_color(PyUILineObject* self, PyObject* value, void* closure) {
         return -1;
     }
     self->data->setColor(color->data);
+    self->data->markDirty(); // #291: color change
     return 0;
 }
 
@@ -486,6 +489,7 @@ int UILine::set_thickness(PyUILineObject* self, PyObject* value, void* closure)
     }
 
     self->data->setThickness(thickness);
+    self->data->markDirty(); // #291: visual change
     return 0;
 }