feat: redesign Insert tab and fix all warnings

- Redesign Insert ribbon tab with 6 groups matching screenshot layout:
  Reference (Attach/Clip/Adjust large + Underlay Layers/Frames/Snap small),
  Point Cloud (Attach), Block (Multi-View Block/Insert large + Create/Edit/Base small),
  Attributes (Define/Edit large + Manage/Sync small),
  Import (Import/Land XML), Content (Content Browser/Design Center)
- Remove Primitives group and dead box_prim/clear/cylinder/sphere modules
- Fix all compiler warnings with #[allow(dead_code)] on unused-but-intentional items

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/icons/attdef.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
+  <!-- Tag shape -->
+  <path d="M3 3 h8 l10 9 -10 9 H3 Z"/>
+  <circle cx="8" cy="12" r="1.5" fill="currentColor" stroke="none"/>
+  <!-- "A" attribute letter -->
+  <text x="5" y="14" font-size="6" fill="currentColor" stroke="none" font-weight="bold">A</text>
+  <!-- Plus (define/create) -->
+  <line x1="19" y1="3" x2="19" y2="9"/>
+  <line x1="16" y1="6" x2="22" y2="6"/>
+</svg>