The standard library special-cases `run` for MainActor alone, so code
shared between Darwin and Android could not hop to the platform thread
through a single spelling. Mirrors the existing assumeIsolated helper.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BFRbYavk7m8zHvRHfA5D36