[6] 3D viewport visual polish — local axes and section rendering

[Batuis]

Adds the 3D viewport visual polish layer after PR [5].

Includes:

• selected-member local-axis triads using computeLocalAxes3D as-is
• local axes display mode: Always / When selected / Never, default When selected
• persistence for the local-axis display mode
• compact wireframe ↔ sections viewport toggle
• section/profile render polish with edges/material updates
• frame + truss section rendering in sections mode
• industrial warehouse fixture render geometry improvements
• generic section orientation fix using local axes
• normal pan/orbit keeps profiles, loads, results, and visual detail visible

Non-scope:

• no solver changes
• no local-axis convention change
• no analytical member offsets
• no shell/CAD work
• no verification/result-value changes

Manual QA passed locally. CI monitoring intentionally skipped because the stack has a known coworker-owned engine CI issue.

🤖 Generated with Claude Code