fix: polish the four newest examples

[TMHSDigital]

Summary

• temp-override-join: coplanar L to three-step staircase; Z-span assert catches no-op override.
• shape-key-blend: Tall key lifts and flares (truncated pyramid); per-vert blend formula check.
• gn-instance-grid: larger cubes, Set Shade Smooth off, corner-instance center probe.
• curve-bevel-arc: thicker bevel, use_fill_caps, topology 1044/1028, X-span assert.

Re-rendered heroes/previews and regenerated gallery pages.

Test plan

• Check-only on Blender 5.1 and 4.4 for all four
• Re-rendered and converted webps
• build_gallery.py to 12 detail pages
• CI smoke jobs green

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