Streaming callers see every terminal event their result reports

[tobert]

execute_with_options_streaming calls on_output once per top-level statement, and kaish -c prints only what on_output delivers. Text that reaches the returned result but not the callback is text that caller never shows. Three paths did that, or ran a program the caller had already cancelled.

An exit statement added its drained stderr to the aggregate but streamed the statement without it:

kaish -c 'exit $(ls /nope; echo 3)'    # printed nothing; now prints the ls error
kaish -c 'echo $(ls /nope; echo 3)'    # already printed it

The watchdog wrote timeout: timed out after ... only into an empty err. A timed-out program that printed any stderr, including sleep: interrupted, exited 124 with no timeout line. The line is now appended after existing stderr, as the timeout builtin does, and streamed, since no statement carries a deadline. Whole-program background jobs receive it through on_output, so their separate timeout handling is removed.

execute_streaming_inner reset the cancel token its caller had just installed. When an embedder or job token fired before the first statement, the reset replaced it and the program ran to completion; echo a; echo b under a cancelled token exited 0. The loop now keeps the installed token and stops at its first checkpoint.

A cancelled call now exits 130 whatever signal ended its child, matching how a timeout exits 124. A cancel that killed a lone external command reported the child's own status, 143 after SIGTERM or 137 after the grace escalated to SIGKILL, because the statement loop's 130 checkpoint runs only between statements. The token decides, not the code, so exit 143 is unchanged. grace_escalation_sigkills_term_trapping_child used 137 as its evidence of escalation; it now expects 130 and keeps the evidence that already proved it, since the child ignores SIGTERM, dies anyway, and not before the grace. If 137 should stay visible to callers, that assertion is the one to revisit.

The new streaming tests use only builtins, so a default cargo test runs them.

This stacks on #445.

Co-Authored-By: DeepSeek V4 Flash noreply@deepseek.com
Co-Authored-By: Claude Opus 5 noreply@anthropic.com

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