test: make SharedWorker admission control deterministic

[aantenore]

Summary

• replace timing-dependent SharedWorker backpressure setup with an active holder barrier
• dispatch four contenders against one shared capacity budget and clean up all admitted work deterministically
• prepare v0.3.0-alpha.2 so the latest prerelease is backed by a retry-free browser gate

Verification

• pnpm check
• pnpm package:smoke
• pnpm test:browser (30/30)
• WebKit admission-control stress (20/20)