Fix main's build, and run a negated body unpublished like a chain operand

[tobert]

Merging #465 and then #466 broke main's build. #465 gave drain_stderr_into a ctx parameter, and #466 added three call sites that use the old signature. Each PR was green against a main that did not have the other.

Passing ctx fixes the build, but it exposed two stderr bugs in background jobs, where a fault under ! interacts with #465's publishing model:

f() { ! [[ 1 -eq abc ]]; }; f &              # the diagnostic reached the job stream twice
f() { ! [[ 1 -eq abc ]]; }; timeout 5 f &    # "<diag>\ntimeout: \n", but the foreground printed "timeout: <diag>"

The ! arm ran its body through execute_stmt_flow, which publishes stderr, and it used all of result.err as the error message. The arm now does what the &&/|| arms do with their left side. The body runs through execute_stmt_flow_dispatch, which does not publish. A fault takes only its unpublished stderr as the message, and every other result is published before the drain.

New job-stderr corpus cases cover a negated builtin error, a negated pipeline, a negated fault with and without earlier published stderr, and a timeout-wrapped fault under ! and &&. Going back to the publishing call fails the timeout case, and using the whole err as the message fails the published-body case. No test pins the drain after the body; every enclosing context also drains per statement.

The fix does not cover a pre-existing ordering bug, which is pinned with an ignored test. When a command's $(…) fails, result.err lists the command's own stderr before the substitution's, while the job stream has them in the order they ran. The && arm has it too, so it gets its own PR.

Reviewed with kaibo (DeepSeek); its timeout and ordering findings reproduced, as above.

🤖 Generated with Claude Code

[tobert]

kaibo review (cast deepseek, deepseek-flash) of the ! arm against the job-stderr publishing model:

• No path through the arm publishes twice, drops stderr, or can trip the publish/drain asserts (split_off leaves stderr_published_len == err.len()).
• Found: in a job, a negated fault's error message was empty, so timeout rendered a bare timeout: . Reproduced; fixed in d3e60f1 by running the body unpublished.
• Found: result.err puts a command's own stderr before its $(…)'s, while the job stream keeps run order; && too. Reproduced; pinned as an ignored test for a follow-up PR.
• Claimed the fault tests could not discriminate the message split; mutation shows reverting it fails the published-body case.

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