Re-Raise the Interrupt Signal and Exempt a Script That Execs Its Process - #4134

Summary

• CODESTYLE.md "Interrupts": after its cleanup, an interrupted script re-raises the signal rather than calling exit 130 or exit 143. A shell still reports 130 or 143. At Ctrl-C, a calling shell's loop now stops instead of starting its next iteration. A launcher such as uv run exits with the status rather than dying by the signal, and the rule says so. The mid-run test asserts the script died by the signal, and starts the script with its interpreter directly.
• "Interrupts in Python": the entry point flushes its output under contextlib.suppress(OSError), restores the default handler with signal.signal(signum, signal.SIG_DFL), and re-sends the signal with os.kill(os.getpid(), signum). The re-raise runs outside every with block and finally clause, and no cleanup on its path relies on an atexit handler or a finalizer. A process the signal does not end, such as PID 1 in a container, then exits 128 plus the signal number. On Windows, where os.kill would end the process with the signal number as its status, it keeps exiting 130.
• "Interrupts in Shell": each trap clears itself with trap - INT or trap - TERM, re-sends the signal with kill -s INT "$$" or kill -s TERM "$$", then ends with exit 130 or exit 143, which runs only where the signal does not end the script, such as PID 1. The EXIT trap is set before the INT and TERM traps, since bash skips it when a TERM trap set ahead of it re-sends SIGTERM.
• "Interrupts" exempts a script that ends by exec-ing its long-running process, such as a container entrypoint or an s6 or runit run script. A background child may be handed across the exec, neither stopped nor waited for, where something else stops it, such as a container's teardown. Anything else the script holds is released before the exec. A script holding nothing else at any point before the exec owes no trap and no signal test. As PID 1, such a script ignores a SIGTERM that arrives before its exec, and the rule says so. "Interrupts in Shell" points at that exemption from its trap and wait bullets.
• python-codestyle and shell-codestyle regenerated through scripts/build_dist.py. python-codestyle's description is unchanged.

Verification

A constructed probe, run under setsid --wait timeout: a non-interactive bash loop runs a child three times, and SIGINT goes to the loop's whole process group, as Ctrl-C does. With bash 5.2, Python 3.13, and uv 0.13:

With SIGTERM sent to the child alone, the loop reads status 143 from both forms. Further probes: a bash TERM trap set before the EXIT trap skips the EXIT trap on its re-sent SIGTERM, and runs it when set after. A Python PID 1 in a PID namespace survives its own re-sent SIGTERM and reaches the 143 fallback. Bash as PID 1 with no EXIT trap survives its own re-sent SIGTERM and runs on, and the trailing exit 143 ends it. Bash as PID 1 with no traps ignores a SIGTERM sent from outside its namespace. build_dist.py --check, the prose gate, and markdownlint pass.

Local review

Each push carries a recorded local pass. The findings those passes raised were fixed on this branch, so none stays open.

Closes on promotion: #4084, #4097

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