Require clean shutdown before protocol completion

[Taz33m]

Summary

• close the engine exactly once before emitting the terminal complete frame
• emit one ADAPTER_ERROR and exit 2 when shutdown fails
• preserve an earlier protocol error when shutdown also fails
• verify the behavior through direct transcripts and a real child-process adapter

Observable change

Only the previously dishonest close-failure path changes: complete is omitted and ADAPTER_ERROR is emitted. Normal and protocol-error transcripts remain byte-for-byte identical.

Verification

• full pytest suite
• black, flake8, mypy, bandit, compileall
• baseline transcript byte comparison
• built-wheel metadata check and isolated qualification smoke: 3/3 fixed, 25/25 traces, 5,000 events, 10/10 capabilities

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Summary by cubic

Enforces a clean shutdown contract for the stdio protocol: complete is sent only after a successful adapter.close(). If shutdown fails, the server sends a single ADAPTER_ERROR, omits complete, and exits with code 2, while preserving any earlier PROTOCOL_ERROR.

• Bug Fixes
  • Close the engine exactly once before emitting complete; on close() failure, emit ADAPTER_ERROR and exit 2.
  • Preserve a prior PROTOCOL_ERROR if close() also fails; don’t overwrite the terminal error.
  • Centralize error writes via _write_error and update docs/tests; normal and protocol-error transcripts remain unchanged.

^{Written for commit 927a330. Summary will update on new commits.}