runtime-node: prevent duplicate ProcessIO writes under backpressure

[bbopen]

Summary

• fix ProcessIO backpressure handling so writes are not replayed when stdin.write() returns false
• treat write() === false as accepted-by-buffer and pause only subsequent writes until drain
• add regression tests that fail if writes are duplicated during drain/backpressure cycles

Why this matters

Node stream semantics guarantee that write() returning false means the chunk was accepted and buffered. Re-queueing that same chunk can duplicate protocol frames and cause incorrect request delivery.

Validation

• npx eslint src/runtime/process-io.ts test/transport.test.ts
• npm run typecheck
• npm test -- test/transport.test.ts
• npm test -- test/runtime_node.test.ts

Closes #144

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📥 Commits

Reviewing files that changed from the base of the PR and between d0a2513 and 481af4d.

📒 Files selected for processing (2)

• src/runtime/process-io.ts
• test/transport.test.ts

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch codex/144-processio-backpressure-no-duplicate-writes

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