Feat/stairs fence update

[sudhir9297]

What does this PR do?

Adds a new Fence build element and introduces major stair system improvements, aligning both with plan3D behavior and the project’s 3D editor workflows.

This PR includes

Fence system

• New fence scene node/schema with parametric properties
• 3D fence drawing in build mode with wall-like snapping
• Fence rendering in the viewer/editor
• Selection, move, delete, duplicate, and scene-tree support
• Sidebar panel for editing fence parameters
• Linked endpoint behavior for connected fences (live updates on move)
• Toolbar/sidebar integration with fence icon

Stair system

• Stair-level railing controls with rendering for straight stairs
• Fixes for railing mapping and landing behavior
• Railings update live while editing
• New curved and spiral stair types with full rendering + controls
• Fit To Floor support for curved stairs and improved panel logic
• Spiral stair features (center column, supports, optional top landing)
• Fixed stair duplication and improved stability
• Runtime crash fixes for invalid wall children data

---

How to test (summary)

• Draw and edit fences; verify snapping, parameters, and linked behavior
• Test fence actions (move, duplicate, delete) and scene-tree presence
• Create and modify stairs (straight, curved, spiral); verify railings and controls
• Switch stair types live and test duplication/movement
• Validate no regressions in wall/door/window interactions