ci(audience): add linux-build-smoke build-validation tier

[ImmutableJeffrey]

Summary

• Adds new linux-build-smoke job to test-audience-sample-app.yml covering 6 cells (3 Unity x 2 backends), running in parallel.
• Introduces LinuxSmokeRunner MonoBehaviour and LinuxSmokeBuilder editor builder; the builder generates an empty single-runner scene at build time and produces a StandaloneLinux64 player.
• Smoke initialises ImmutableAudience, sends linux_smoke_ci_marker, awaits FlushAsync, exits 0 on success or 1 on any error.
• playmode-linux job left in place; deletion is a separate follow-up after the lavapipe experiment lands.

Test plan

• linux-build-smoke matrix runs on this PR for all 6 cells.
• Each cell completes in under 15 minutes on cold cache.
• Smoke artifact uploaded for at least one passing cell; log shows [LinuxSmoke] OK. line.
• Mutate LinuxSmokeRunner locally to throw in Start; push and confirm cell goes red and [LinuxSmoke] Init threw: appears in log; revert before merge.

🤖 Generated with Claude Code

[ImmutableJeffrey]

Closing without merging.

Findings from the run on this branch:

• 4 of 4 linux-build-smoke cells went green after one license-seat-race rerun (cell time 6.5 to 7.5 min, well under the 15 min target).
• Mechanism worked: builder generated a temp scene at build time, smoke runner called Audience.Init plus one Track plus FlushAsync, exit code zero on success.
• One pre-existing Test Results check from the lint workflow stays NEUTRAL on every PR (dorny/test-reporter sees no NUnit XML); not introduced by this PR.

Not pursuing further. The xvfb-based playmode-linux path on chore/sdk-318-linux-playmode-xvfb stays the mechanism for Linux.

Branch and worktree left in place; safe to delete.