feat: Add UIDrawable children collection to Grid

Grid now supports a `children` collection for arbitrary UIDrawable elements
(speech bubbles, effects, highlights, path visualization, etc.) that
automatically transform with the grid's camera (pan/zoom).

Key features:
- Children positioned in grid-world pixel coordinates
- Render after entities, before FOV overlay (proper z-ordering)
- Sorted by z_index, culled when outside visible region
- Click detection transforms through grid camera
- Automatically clipped to grid boundaries via RenderTexture

Python API:
  grid.children.append(caption)  # Speech bubble follows grid camera
  grid.children.append(circle)   # Highlight indicator

Closes #132

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/test_grid_children.py

@@ -0,0 +1,129 @@
+#!/usr/bin/env python3
+"""Test Grid.children collection - Issue #132"""
+import mcrfpy
+from mcrfpy import automation
+import sys
+
+def take_screenshot(runtime):
+    """Take screenshot after render completes"""
+    mcrfpy.delTimer("screenshot")
+    automation.screenshot("test_grid_children_result.png")
+
+    print("Screenshot saved to test_grid_children_result.png")
+    print("PASS - Grid.children test completed")
+    sys.exit(0)
+
+def run_test(runtime):
+    """Main test - runs after scene is set up"""
+    mcrfpy.delTimer("test")
+
+    # Get the scene UI
+    ui = mcrfpy.sceneUI("test")
+
+    # Create a grid without texture (uses default 16x16 cells)
+    print("Test 1: Creating Grid with children...")
+    grid = mcrfpy.Grid(grid_size=(20, 15), pos=(50, 50), size=(320, 240))
+    grid.fill_color = mcrfpy.Color(30, 30, 60)
+    ui.append(grid)
+
+    # Verify entities and children properties exist
+    print(f"  grid.entities = {grid.entities}")
+    print(f"  grid.children = {grid.children}")
+
+    # Test 2: Add UIDrawable children to the grid
+    print("\nTest 2: Adding UIDrawable children...")
+
+    # Speech bubble style caption - positioned in grid-world pixels
+    # At cell (5, 3) which is 5*16=80, 3*16=48 in pixels
+    caption = mcrfpy.Caption(text="Hello!", pos=(80, 48))
+    caption.fill_color = mcrfpy.Color(255, 255, 200)
+    caption.outline = 1
+    caption.outline_color = mcrfpy.Color(0, 0, 0)
+    grid.children.append(caption)
+    print(f"  Added caption at (80, 48)")
+
+    # A highlight circle around cell (10, 7) = (160, 112)
+    # Circle needs center, not pos
+    circle = mcrfpy.Circle(center=(168, 120), radius=20,
+                           fill_color=mcrfpy.Color(255, 255, 0, 100),
+                           outline_color=mcrfpy.Color(255, 255, 0),
+                           outline=2)
+    grid.children.append(circle)
+    print(f"  Added highlight circle at (168, 120)")
+
+    # A line indicating a path from (2,2) to (8,6)
+    # In pixels: (32, 32) to (128, 96)
+    line = mcrfpy.Line(start=(32, 32), end=(128, 96),
+                       color=mcrfpy.Color(0, 255, 0), thickness=3)
+    grid.children.append(line)
+    print(f"  Added path line from (32,32) to (128,96)")
+
+    # An arc for range indicator at (15, 10) = (240, 160)
+    arc = mcrfpy.Arc(center=(240, 160), radius=40, start_angle=0, end_angle=270,
+                     color=mcrfpy.Color(255, 0, 255), thickness=4)
+    grid.children.append(arc)
+    print(f"  Added range arc at (240, 160)")
+
+    # Test 3: Verify children count
+    print(f"\nTest 3: Verifying children count...")
+    print(f"  grid.children count = {len(grid.children)}")
+    assert len(grid.children) == 4, f"Expected 4 children, got {len(grid.children)}"
+
+    # Test 4: Children should be accessible by index
+    print("\nTest 4: Accessing children by index...")
+    child0 = grid.children[0]
+    print(f"  grid.children[0] = {child0}")
+    child1 = grid.children[1]
+    print(f"  grid.children[1] = {child1}")
+
+    # Test 5: Modify a child's position (should update in grid)
+    print("\nTest 5: Modifying child position...")
+    original_pos = (caption.pos.x, caption.pos.y)
+    caption.pos = mcrfpy.Vector(90, 58)
+    new_pos = (caption.pos.x, caption.pos.y)
+    print(f"  Moved caption from {original_pos} to {new_pos}")
+
+    # Test 6: Test z_index for children
+    print("\nTest 6: Testing z_index ordering...")
+    # Add overlapping elements with different z_index
+    frame1 = mcrfpy.Frame(pos=(150, 80), size=(40, 40))
+    frame1.fill_color = mcrfpy.Color(255, 0, 0, 200)
+    frame1.z_index = 10
+    grid.children.append(frame1)
+
+    frame2 = mcrfpy.Frame(pos=(160, 90), size=(40, 40))
+    frame2.fill_color = mcrfpy.Color(0, 255, 0, 200)
+    frame2.z_index = 5  # Lower z_index, rendered first (behind)
+    grid.children.append(frame2)
+    print(f"  Added overlapping frames: red z=10, green z=5")
+
+    # Test 7: Test visibility
+    print("\nTest 7: Testing child visibility...")
+    frame3 = mcrfpy.Frame(pos=(50, 150), size=(30, 30))
+    frame3.fill_color = mcrfpy.Color(0, 0, 255)
+    frame3.visible = False
+    grid.children.append(frame3)
+    print(f"  Added invisible blue frame (should not appear)")
+
+    # Test 8: Pan the grid and verify children move with it
+    print("\nTest 8: Testing pan (children should follow grid camera)...")
+    # Center the view on cell (10, 7.5) - default was grid center
+    grid.center = (160, 120)  # Center on pixel (160, 120)
+    print(f"  Centered grid on (160, 120)")
+
+    # Test 9: Test zoom
+    print("\nTest 9: Testing zoom...")
+    grid.zoom = 1.5
+    print(f"  Set zoom to 1.5")
+
+    print(f"\nFinal children count: {len(grid.children)}")
+
+    # Schedule screenshot for next frame
+    mcrfpy.setTimer("screenshot", take_screenshot, 100)
+
+# Create a test scene
+mcrfpy.createScene("test")
+mcrfpy.setScene("test")
+
+# Schedule test to run after game loop starts
+mcrfpy.setTimer("test", run_test, 50)