Make Bun exec output reliable in the full runtime suite

[taras]

Problem

The Bun 1.3.14 runtime gate can time out packages/core/tests/execute.test.ts
D3b only when it runs inside the complete 168-file suite.

Observed while verifying #340 on 2026-08-05:

• npx --yes --package=bun@1.3.14 bun run test:bun
  • D3b failed after 30,047 ms.
  • The command produced no partial stdout and was reported as
    exec(bash) timed out after 30000ms.
  • Suite result: 2,366 passed / 1 failed.
• Immediately afterward,
  npx --yes --package=bun@1.3.14 bun test packages/core/tests/execute.test.ts
  • D3b passed in 59 ms.
  • The file passed 45 / 45.

The failing test executes a command that prints partial and exits non-zero.
The difference between the full-suite and focused runs suggests suite-level
process or pipe starvation, leaked runtime state, or resource contention. Do
not raise the timeout or waive the Bun gate without locating the lifetime
problem.

Acceptance

• Repeated Bun 1.3.14 full-suite runs complete D3b without timing out.
• D3b still proves stdout is preserved before the printed error from a non-zero
  exit.
• The regression test fails under the diagnosed leaked-state or starvation
  mutation, rather than passing only because its timeout increased.
• Deno and Node behavior remain unchanged.

Scope

This was discovered during #340 verification. #340 does not change execution,
process handling, D3b, or Bun adapters; this issue owns the flake independently.