fix(flame_3d): Remove duplicate baseColor multiply in ambient calculation

[agnath18K]

Description

Fixes #3813

The ambient calculation in spatial_material.frag multiplies baseColor twice:

vec3 baseAmbient = vec3(0.03) * baseColor * ...;  // already has baseColor
vec3 ambient = baseAmbient * baseColor * ao;       // multiplied again

This produces ambient ∝ baseColor² instead of baseColor. Dark surfaces get disproportionately crushed under ambient light — a non-linear distortion that diverges from the LearnOpenGL PBR reference cited in the shader header.

Removed the extra * baseColor on the second line. Recompiled shaderbundle.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Shader math correction — not feasible to unit test without a GPU context.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.