Tweak BroadcastChannel lifetimes

[Jarred-Sumner]

What does this PR do?

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

[robobun]

^{Updated 11:09 PM PT - May 23rd, 2025}

❌ @Jarred-Sumner, your commit e6f0c54 has 3 failures in Build #17293:

• test/js/node/test/parallel/test-cluster-worker-kill-signal.js - code 1 on 🐧 3.21 x64-baseline
• test/napi/node-napi.test.ts - error: addresssanitizer on 🐧 12 x64-asan
• test/js/bun/spawn/spawn.test.ts - 1 failing on 🪟 2019 x64
• test/js/bun/spawn/spawn.test.ts - 1 failing on 🪟 2019 x64-baseline

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🧪   To try this PR locally:

bunx bun-pr 19881

That installs a local version of the PR into your bun-19881 executable, so you can run:

bun-19881 --bun