chore(audience-sample): drop Standalone default quality to Very Low

[ImmutableJeffrey]

Summary

• One-line change to QualitySettings.asset: m_PerPlatformDefaultQuality.Standalone: 5 -> 0.
• Standalone Linux CI cells boot at quality level 0 (Very Low): no shadows, no anisotropic, no antialiasing, no realtime reflection probes, no soft particles or vegetation.
• Standalone Windows and macOS cells also drop to Very Low. Their CI cells are GPU-accelerated and already finish in 2 to 4 min, so no regression expected.

Experiment success criteria

• Unity 6 Linux Mono cell wall time under 22 min (currently around 27 min on baseline).
• Unity 2021.3 Linux cells stay at or below their current 4 min.
• All 4 Linux PR cells go green (no test failures introduced by the quality drop).

🤖 Generated with Claude Code