runtime-node: surface explicit TYWRAP_CODEC_MAX_BYTES startup errors

[bbopen]

Summary

• include captured stderr context in ProcessIO stdin/write stream errors so startup failures retain root-cause diagnostics
• add a deterministic transport regression test for write-time EPIPE/error paths carrying TYWRAP_CODEC_MAX_BYTES details
• add adversarial coverage for invalid TYWRAP_CODEC_MAX_BYTES values

Testing

• npx eslint src/runtime/process-io.ts test/transport.test.ts test/adversarial_playground.test.ts
• npm run typecheck
• npm test -- test/transport.test.ts
• npm test -- test/runtime_node.test.ts -t "TYWRAP_CODEC_MAX_BYTES"
• TYWRAP_ADVERSARIAL=1 npm test -- test/adversarial_playground.test.ts -t "invalid TYWRAP_CODEC_MAX_BYTES"

Closes #52

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 2cf0e27 and 139cffa.

📒 Files selected for processing (3)

• src/runtime/process-io.ts
• test/adversarial_playground.test.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/52-codec-max-bytes-explicit-env-error

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