[aliorhun]

What this adds

The main viewer's Parts in this step panel now includes a compact Placement preview so you can see where the current-step pieces go without opening the full Instructions view.

Behavior

• renders a small 3D exploded placement preview for the current step
• uses the same green placement arrows as Instructions
• focuses the camera on the actual placement area instead of the whole model
• supports multiple pieces in the same step
• for underside-only attachments, automatically shows the mini preview from below
• for mixed top/bottom attachments, uses a shallower neutral viewing angle
• keeps the regular BOM chips above the preview
• updates automatically on Prev/Next/first/last step navigation
• keeps the mobile panel compact

Implementation

• uses a small reusable offscreen Three.js renderer
• reuses the existing build-step, exploded-placement, attachment-direction, and arrow logic
• resets/disposes preview resources when a new model is opened
• does not require opening or pre-generating the full Instructions booklet

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