Art(shader): Emissive-Gloam master shader (in-shader flicker cadence)

ProjectM/EmissiveGloam — URP HLSL, HDR emission (Bloom-catching), with the
readability 'steady=true / flicker=false' channel as a material param
(_FlickerAmount), no script. Applied to the staging flora bulbs (cold, flicker).
The other two master shaders already exist (Lit URP graph, AnimatedLitShader).

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

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