Fix particle demo dependencies and guards

[lyonsno]

Summary

• load the missing core engine scripts for the particle demo and align the legacy SEC3ENGINE namespace
• add camera factory helpers and canvas registration expected by the old demo
• guard particle system setup against absent scene/light resources to avoid WebGL errors

Testing

• Playwright smoke test of particleDemo.html

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

renderParticles bails out when no light is present

The new guard in renderParticles returns whenever no light with cascade framebuffers is found, but particleDemo.html/Sec3Engine/demos/ParticleDemo.js never create a light and drawScene calls system.draw() without passing one. In that scenario getActiveLight() yields null, this guard trips, and the demo never issues any draw calls, leaving the canvas blank even though the rest of the setup succeeds. Rendering should handle the no-light case instead of exiting early or the demo needs to provide a light.

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[lyonsno]

@codex If the light isn't being added, why was this working originally?