The stdin destroy (child.stdin?.destroy() right after spawn) previously had no
in-tree coverage. A program that reads fd 0 now sees EOF immediately; without
the destroy it blocks and the run would hang to maxWallMs as a timeout —
verified fail-before by disabling the destroy (the test turns red at the wall
ceiling) and restoring it (green). The _str rebind regression was attempted but
is not viable: the success path's done-frame serialization reaches str
transitively through _encode_json_plain, which the README Known Limitations
already records as the accepted success-to-exception residual, so any rebind
test trips that documented residual before send_done's bound _str.
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