ci(audience): swap llvmpipe -> lavapipe (Vulkan) for playmode-linux

[ImmutableJeffrey]

Summary

• Stacks on top of chore/sdk-318-linux-playmode-xvfb. Diff is lavapipe-only.
• Installs mesa-vulkan-drivers and vulkan-tools inside the unityci/editor container before the xvfb-driven Unity run.
• Sets MESA_VK_DEVICE_SELECT=lvp so multiple Vulkan ICDs do not race.
• Drops -force-glcore from the Unity command line so Unity 6 picks its native Vulkan path.
• Uploads vulkaninfo.txt as a CI artifact for post-run device confirmation.

Experiment success criteria (from spec)

• Unity 6 Mono cell wall time under 15 min: declare win, propose merge into SDK-318 (and onward to main).
• Unity 6 Mono cell wall time 15 to 22 min: marginal; close PR with measurements.
• Unity 6 Mono cell wall time above 22 min or any test failure not present on the SDK-318 baseline: regression; close PR with findings.
• Unity 2021.3 cells stay at or below their current 4 min on the SDK-318 baseline.

Test plan

• All 6 playmode-linux cells run on this PR.
• vulkaninfo.txt artifact present on at least one cell and shows llvmpipe (LLVM ...) as the selected device.
• Compare cell wall times against the most recent SDK-318 same-day run; record numbers in PR comment.

🤖 Generated with Claude Code

[ImmutableJeffrey]

Closing without merging.

Findings from this branch's run:

• Lavapipe was installed inside the container but Unity did not pick it up. Cell logs show Renderer: llvmpipe (LLVM 15.0.7, 256 bits), meaning Unity 6's Vulkan init failed and fell back to OpenGL on llvmpipe.
• Likely cause: Vulkan ICD path inside the unityci/editor container differs from Unity's search path. MESA_VK_DEVICE_SELECT=lvp only matters once a Vulkan loader actually finds an ICD; on this image it never did.
• Wall time on Unity 6 cells was within margin of the SDK-318 baseline (28 to 30 min vs 27 min). Tests passed/failed identically to baseline (39 passed, 1 failed, the 1 failure being SDK-341 which is tracked separately).
• Even if the ICD discovery were fixed, software Vulkan still rasterises on CPU; the floor is the same as software OpenGL, and the speedup we needed is unlikely.

Not pursuing further. xvfb + llvmpipe on chore/sdk-318-linux-playmode-xvfb remains the Linux mechanism.