Use hand-made screenshots for the two 3D Color examples

ColorExplorer3D and CrossFadeRgb3D are WebGL/3D tools whose rendered
output is non-deterministic, so each CI capture produced a slightly
different poster and the gallery bot committed it on every run. Pin both
with preview.json {"skip": true} and commit a static screenshot.png for
each (captured over a local server so their assets load); the gallery's
thumbnail fallback uses screenshot.* when no generated preview exists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jonfroehlich