Module rendering/picking/point/shaders

GLSL3 picking shader source for points + ShaderSource record.

Mirrors the visual point shader in rendering/materials/point/shader-glsl.ts with three picking-specific additions:

• uNodeId uniform + vNodeId / vElementId varyings, written into the RGBA32F pick buffer as (nodeId, elementId-low16, brightness, elementId-high16).
• 80%-radius truncation so the pick footprint tracks the visible disc while staying slightly biased toward the bright core.
• Brightness-as-depth so the brightest overlapping fragment wins the depth test (matters for hover-through-translucent point stacks).

Source-of-truth for GLSL3; the WebGPU counterpart lives in ./pick.tsl and is referenced through the ShaderSource.webgpu factory below.