When `--capture` is active, stdout and stderr are written to separate
temporary files containing raw, unannotated output. A separate lifecycle
file captures all I: messages.

When set, stdout / stderr is accumulated in memory and replayed after
exit preceded by a section header. When `--capture` is active, writes
into the files happen immediately instead of deferred.

Extract human-readable messages from JSON log lines, replacing raw JSON
with the message field value while preserving cg's annotation format;
non-JSON lines pass through unchanged.

Exercise the compiled binary end-to-end using txtar scripts.