fix(interpreter): exec inside a subshell redirects the subshell

[chaliy]

Requested by Михайло · project thread

Stacked on #2610 — review that one first; this PR's own commit is the second one.

What changed

An exec redirect set inside a subshell now applies to the subshell's output.
Output the subshell wrote before the exec still goes to the caller, as in
bash, and a write to a descriptor the subshell closed names the command that
tried.

Why

The redirect applied to nothing at all. Only the outermost command loop routed
the exec fd table, and a subshell saves and restores its own table, so by the
time control was back at that loop the subshell's redirect was already gone.

( exec > log.txt; echo inside )
echo sep
cat log.txt

	output
bash 5.2	sep, inside
bashkit before	inside, sep — and log.txt empty
bashkit after	sep, inside

( exec 2>err.txt; ls /nope ) lost the capture the same way: the error went to
the caller's stderr and err.txt stayed empty.

Before / After

( echo pre; exec > log.txt; echo post ); echo sep; cat log.txt

	output
bash 5.2	pre, sep, post
bashkit before	pre, post, sep — log.txt empty
bashkit after	pre, sep, post

( exec 1>&- ; echo x )

bash and bashkit now both say
bash: line 1: echo: write error: Bad file descriptor and exit 1.

Risk

Low, and contained to the subshell boundary. The routing helper gained a
"keep" floor so the split between what the caller sees and what the redirect
takes does not depend on whether a streaming callback is attached — the
in-process exec() path and the CLI now agree. Two interpreter fields track
where the enclosing sequence stood when exec installed its target, and the
name of the last simple command, for a diagnostic raised after that command
returned.

An exec redirect inside a bash -c child still does not take effect;
L-PROC-001 is updated to say so.

Checklist

• cargo test -p bashkit --lib (2920 passed)
• cargo test -p bashkit --test integration (1376 passed)
• 3 new spec cases and 4 integration tests, 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, limitations.md L-PROC-001)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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