fix: Improve item surface placement, floorplan resizing, and placement preview stability

[sudhir9297]

What does this PR do?

This PR improves placement behavior, floorplan editing workflows, and preview stability across item placement, slabs, ceilings, and spawn handling.

It enables:

• Placement on valid upward-facing GLB surfaces
• Better handling of rugs/mats and other low-profile surfaces
• Accurate preview bounds and dimension feedback
• Edge resizing for slabs, ceilings, and holes in floorplan
• Full spawn/player interaction support in 2D floorplan
• More stable placement previews and overlay rendering

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Item Surface Placement

• Added support for placing items on valid upward-facing GLB surfaces:
  • No longer limited to assets with explicit asset.surface
• Added low-profile surface handling:
  • Rugs/mats can visually receive placement
  • Do not become parent surfaces
  • Do not block normal floor placement
• Improved upward-surface detection using world normals.

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Placement Preview & Stability

• Fixed placement preview bounds and dimension labels:
  • Now use actual GLB mesh bounds instead of fallback dimensions
• Fixed floor placement cursor height after switching away from:
  • Ceiling placement
  • Item-surface placement
• Fixed detach behavior:
  • Returning to floor resets preview/draft item to y = 0
• Reworked placement bounding boxes and guide-line geometry:
  • Prevents WebGPU vertex-buffer errors during drag/collision
• Removed selected-item height annotation line beside placed GLB items.

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Floorplan Polygon Editing

• Added edge-resize handles for:
  • Slabs
  • Ceilings
  • Polygon holes
• Added shared polygon edge-drag support.
• Updated slab and ceiling editors to support edge movement.

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Spawn / Player Support (Floorplan)

• Added spawn/player support in floorplan:
  • Render
  • Select
  • Focus
  • Move
  • Delete
  • Live transform preview
  • Action-menu anchoring

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Floorplan Overlay & Interaction Fixes

• Fixed action menu positioning under rotated floorplan/building views.
• Updated SVG coordinate handling and rotated view-box projection:
  • Cursor overlays align correctly
  • Action menus align correctly

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Core Changes

• Added:
  • isLowProfileItemSurface
  • LOW_PROFILE_ITEM_SURFACE_MAX_HEIGHT
• Updated spatial grid validation:
  • Ignores low-profile surfaces during floor checks

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Editor Improvements

• Updated placement coordinator:
  • Better preview geometry
  • Better dimension labels
  • Improved transitions between floor, ceiling, and surface placement
• Expanded floorplan interaction system for:
  • Spawn nodes
  • Polygon edge resizing