Highlight incomplete operator nodes in tree and viewport

[kmatzen]

Summary

• Tree UI: Incomplete operators (and ancestors up to root) show red tint, ⚠ warning icon, and auto-expand to reveal urgent "needs shape" placeholder slots
• Slot preservation: Deleting a boolean child inserts a disabled _empty placeholder so the remaining operand keeps its position (A/B order maintained)
• Partial rendering: Incomplete trees now render available geometry instead of blanking the viewport
• Viewport tinting: Shapes from collapsed incomplete booleans tint orange via a generated sdfWarn() GLSL function that preserves ancestor transforms for correct spatial localization

Test plan

• Create a subtract with two children, delete the first — verify the second stays in its slot and a placeholder appears in the first slot
• Verify the warning icon and red tint propagate up to ancestor operators in the tree
• Verify incomplete nodes auto-expand to show missing slots
• Build union(box, subtract(sphere)), delete the sphere's sibling — verify only the sphere tints orange in the viewport while the box stays blue
• Add a translate wrapping the incomplete subtree — verify the orange tint appears at the correct transformed position
• Drop a shape onto a placeholder slot — verify it replaces the placeholder and warnings clear
• Run npx vitest run — all 81 tests pass

🤖 Generated with Claude Code