fix(interpreter): exec N>&- closes the descriptor for real

[chaliy]

Requested by Михайло · project thread

What changed

exec 1>&- and exec 2>&- now close the descriptor instead of quietly
reverting to the caller's stream. A later write to a closed descriptor is
dropped, the command reports write error: Bad file descriptor under its own
name, and its status is 1. When fd 2 is closed too, the report is swallowed but
the status still fails.

Why

Scripts use exec 1>&- to prove nothing writes to stdout afterwards. Bashkit
printed the output and succeeded, so such a script passed in bashkit and failed
in bash.

Before / After

Script:

exec 1>&-
echo x

	stdout	stderr	status
bash 5.2	(empty)	line 2: echo: write error: Bad file descriptor	1
bashkit before	x	(empty)	0
bashkit after	(empty)	bash: line 2: echo: write error: Bad file descriptor	1

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

Risk

Low. Confined to the exec fd table: a new FdTarget::Closed variant that only
exec N>&- produces. Everything else routes as before.

L-PROC-001 now also records that an exec redirect inside a subshell or a
bash -c child does not take effect, since only the outermost command loop
routes the exec fd table. That is pre-existing behavior, found while testing
this, and not changed here.

Checklist

• cargo test -p bashkit --lib (2859 passed)
• cargo test -p bashkit --test integration (1355 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: 2016 KB, inside the two-MiB TM-DOS-020 budget
• Knowledge updated (log.md, limitations.md)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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