fix(interpreter): a failed arithmetic expansion does not truncate the script

[chaliy]

Requested by Михайло · project thread

What changed

A failed $(( )) now fails its own command and nothing more. It still ends an
enclosing subshell, which is what bash does, but it no longer stops the script.

Why

Follow-up to #2604. Reporting arithmetic errors reused the abort path set -u
needs, so the first division by zero silently truncated the run — the error was
printed and everything after it was skipped, with the script exiting 0.

Before / After

echo before
echo $((1/0))
echo "rc=$?"
echo after

	output	status
bash 5.2	before, the error, rc=1, after	0
bashkit before	before, the error — nothing else ran	0
bashkit after	same as bash	0

A subshell still stops, as in bash:

( echo before; echo $((1/0)); echo never ); echo "rc=$?"

gives before and rc=1 in both shells — never does not run.

Risk

Low. The failure carries ControlFlow::Return(1) exactly as before, so every
subshell, function and $( ) boundary behaves as it did; one flag tells the
outermost script loop that this particular Return(1) came from arithmetic and
is not a reason to stop. set -u still ends the script.

Checklist

• cargo test -p bashkit --lib (2920 passed)
• cargo test -p bashkit --test integration (1368 passed)
• New spec case for the script-continues path, verified against bash 5.2
• bash_comparison differential tests against real bash
• cargo clippy -p bashkit --all-targets -- -D warnings
• cargo fmt --all --check
• cargo test -p bashkit-cli (56 passed)
• just check-okf
• Stack probe: 1900 KB, inside the two-MiB TM-DOS-020 budget
• Knowledge updated (log.md)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	c03e4b9	Commit Preview URL Branch Preview URL	Oct 08 2026, 12:26 AM

[chaliy]

Closing as superseded: main now handles arithmetic failures through the line-abort path (#2604 and follow-ups), and this PR's new spec case (echo $((1/0)) then rc=1, script continues) passes on main unchanged. Subshell, brace group, function and if bodies also match bash 5.2 on main.

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