ci(audience): build SwiftShader Vulkan ICD for Unity 6 Linux PlayMode

[ImmutableJeffrey]

Summary

• Builds SwiftShader from source inside the unityci/editor container during the playmode-linux step.
• Sets VK_ICD_FILENAMES to the produced vk_swiftshader_icd.json so the Vulkan loader picks it.
• Drops -force-glcore from the Unity command so Unity 6 tries Vulkan first.
• Uploads vulkaninfo.txt for post-run engagement confirmation.

Engagement check

The Unity log's Renderer: line tells us whether SwiftShader engaged:

• SwiftShader Device -> SwiftShader Vulkan engaged. Compare cell wall times to baseline.
• llvmpipe (LLVM ...) -> Vulkan init still failed and Unity fell back to OpenGL. Same outcome as the closed lavapipe PR. Close this PR with the finding.

Experiment success criteria

• Unity 6 Linux Mono cell wall time under 18 min (currently around 27 min) and vulkaninfo shows SwiftShader as the selected device. Both required.
• If only the wall-time bar is met but Renderer is still llvmpipe, the speedup is from something incidental and we close.

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