fix(interpreter): redirection errors read like bash

[chaliy]

Requested by Михайло · project thread

What changed

A redirection that cannot be opened now reports the way bash does:

• the diagnostic carries the line the redirect is on
• the in-memory backend's is a directory is capitalized to the strerror text bash prints
• a target written with a trailing slash is a directory, so echo x > nodir/ fails instead of creating a file named nodir

Why

A script that redirects into a path it did not create could not tell which
redirect failed, and the trailing-slash case failed silently in the worst
direction: bashkit created a file where the script meant a directory, so
later reads found data in a place bash would have left empty.

Before / After

echo a > f.txt
set -o noclobber
echo b > f.txt     # line 3
set +o noclobber
mkdir d
echo x > d         # line 6
echo y > nodir/    # line 7

line	bash 5.2	bashkit before	bashkit after
3	line 3: f.txt: cannot overwrite existing file	bash: f.txt: cannot overwrite existing file	bash: line 3: f.txt: cannot overwrite existing file
6	line 6: d: Is a directory	bash: d: is a directory	bash: line 6: d: Is a directory
7	line 7: nodir/: Is a directory, status 1	(no error; created a file nodir)	bash: line 7: nodir/: Is a directory, status 1

bash names the running script where bashkit says bash:; the interpreter does
not track a script path, which is a separate known gap.

Risk

Low, but the message text is user-visible. Three existing tests asserted the
old unprefixed form and were updated. The fuzz leak detector's template match
keys on the bash: prefix and the reason suffix, both unchanged, so it still
recognizes these lines as shell echoes rather than internal leaks.

Checklist

• cargo test -p bashkit --lib (2859 passed)
• cargo test -p bashkit --test integration (1356 passed)
• 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, just check-doc-links
• Stack probe: 1992 KB, inside the two-MiB TM-DOS-020 budget
• Knowledge updated (log.md)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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Generated by Claude Code

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