Add regression tests for Frame.children mutation and parent=None removal

frame_children_mutation_test: validates remove(), property mutation via
stored refs, fill_color persistence, iteration mutation, pop(), and
while-loop clearing — with visual screenshot verification.

parent_none_removal_test: validates that setting .parent = None removes
children from Frame.children and scene.children, Entity.grid = None
removal, and Grid overlay children removal.

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

tests/regression/frame_children_mutation_test.py

@@ -0,0 +1,176 @@
+"""Test Frame.children mutation behaviors reported as potential bugs.
+
+Three failure modes reported:
+1. remove() on child doesn't visually update
+2. Property mutation on stored references doesn't render
+3. Clearing children via while-loop doesn't work
+
+This test validates the data model. Visual screenshots require
+a timer-based approach with mcrfpy.step() in headless mode.
+"""
+import mcrfpy
+from mcrfpy import automation
+import sys
+
+PASS = True
+results = []
+
+def check(name, condition):
+    global PASS
+    if not condition:
+        PASS = False
+        results.append(f"  FAIL: {name}")
+    else:
+        results.append(f"  pass: {name}")
+
+# ============================================================
+# Test 1: remove() actually modifies the children collection
+# ============================================================
+
+scene = mcrfpy.Scene("test")
+mcrfpy.current_scene = scene
+
+box = mcrfpy.Frame(pos=(10,10), size=(200,200),
+                    fill_color=mcrfpy.Color(40,40,40))
+scene.children.append(box)
+
+cap1 = mcrfpy.Caption(text="child1", pos=(5,5))
+cap2 = mcrfpy.Caption(text="child2", pos=(5,25))
+cap3 = mcrfpy.Caption(text="child3", pos=(5,45))
+box.children.append(cap1)
+box.children.append(cap2)
+box.children.append(cap3)
+
+check("1a: initial children count is 3", len(box.children) == 3)
+
+# Remove middle child
+box.children.remove(cap2)
+check("1b: children count after remove is 2", len(box.children) == 2)
+check("1c: first child is still cap1", box.children[0].text == "child1")
+check("1d: second child is now cap3", box.children[1].text == "child3")
+
+# ============================================================
+# Test 2: Property mutation on stored references
+# ============================================================
+
+stored_ref = box.children[0]
+check("2a: stored ref text matches", stored_ref.text == "child1")
+stored_ref.text = "MUTATED"
+check("2b: mutation via stored ref visible via children[0]",
+      box.children[0].text == "MUTATED")
+
+# Verify identity: stored ref should point to same C++ object
+check("2c: stored ref identity (same text after re-read)",
+      stored_ref.text == box.children[0].text)
+
+# ============================================================
+# Test 3: fill_color mutation on child persists
+# ============================================================
+
+cap_direct = box.children[1]
+original_color = cap_direct.fill_color
+cap_direct.fill_color = mcrfpy.Color(255, 0, 0, 255)
+reread_color = box.children[1].fill_color
+check("3a: fill_color mutation persists (r)",
+      reread_color.r == 255)
+check("3b: fill_color mutation persists (g)",
+      reread_color.g == 0)
+
+# ============================================================
+# Test 4: Mutation via iteration
+# ============================================================
+
+for child in box.children:
+    child.text = "ITER_" + child.text
+
+check("4a: mutation via iteration on child 0",
+      box.children[0].text == "ITER_MUTATED")
+check("4b: mutation via iteration on child 1",
+      box.children[1].text == "ITER_child3")
+
+# ============================================================
+# Test 5: pop() works
+# ============================================================
+
+popped = box.children.pop()
+check("5a: pop() reduces count", len(box.children) == 1)
+check("5b: popped element has correct text", popped.text == "ITER_child3")
+
+# ============================================================
+# Test 6: while-loop clearing
+# ============================================================
+
+box.children.append(mcrfpy.Caption(text="a", pos=(0,0)))
+box.children.append(mcrfpy.Caption(text="b", pos=(0,0)))
+box.children.append(mcrfpy.Caption(text="c", pos=(0,0)))
+check("6a: re-added children", len(box.children) == 4)
+
+while len(box.children):
+    box.children.pop()
+check("6b: while-loop clearing empties children", len(box.children) == 0)
+
+# ============================================================
+# Test 7: Visual rendering via screenshots
+# ============================================================
+
+scene2 = mcrfpy.Scene("visual")
+parent = mcrfpy.Frame(pos=(10,10), size=(400,300),
+                       fill_color=mcrfpy.Color(40,40,40))
+scene2.children.append(parent)
+
+red_box = mcrfpy.Frame(pos=(10,10), size=(80,80),
+                        fill_color=mcrfpy.Color(255,0,0))
+green_box = mcrfpy.Frame(pos=(100,10), size=(80,80),
+                          fill_color=mcrfpy.Color(0,255,0))
+blue_box = mcrfpy.Frame(pos=(190,10), size=(80,80),
+                         fill_color=mcrfpy.Color(0,0,255))
+
+parent.children.append(red_box)
+parent.children.append(green_box)
+parent.children.append(blue_box)
+
+mcrfpy.current_scene = scene2
+
+# Render a frame, take screenshot with all 3
+mcrfpy.step(0.05)
+automation.screenshot("frame_children_7a_all3.png")
+
+# Remove green box
+parent.children.remove(green_box)
+check("7a: remove green_box reduces count to 2", len(parent.children) == 2)
+
+mcrfpy.step(0.05)
+automation.screenshot("frame_children_7b_no_green.png")
+
+# Mutate red to yellow via stored reference
+red_box.fill_color = mcrfpy.Color(255, 255, 0)
+mcrfpy.step(0.05)
+automation.screenshot("frame_children_7c_yellow.png")
+
+# Mutate via indexed access
+parent.children[0].fill_color = mcrfpy.Color(255, 0, 255)  # magenta
+mcrfpy.step(0.05)
+automation.screenshot("frame_children_7d_magenta.png")
+
+# Clear all
+while len(parent.children):
+    parent.children.pop()
+check("7b: all children cleared", len(parent.children) == 0)
+
+mcrfpy.step(0.05)
+automation.screenshot("frame_children_7e_cleared.png")
+
+# ============================================================
+# Report
+# ============================================================
+
+print("=" * 50)
+print("Frame.children mutation test results:")
+for r in results:
+    print(r)
+print("=" * 50)
+if PASS:
+    print("PASS")
+else:
+    print("FAIL")
+sys.exit(0 if PASS else 1)