fix(parser): {arr[i]} is an fd variable in every redirect position

[chaliy]

Requested by Михайло · project thread

What changed

{arr[i]} is recognised as a redirect's fd variable wherever one may appear,
not only in the close form: exec {A[1]}> file and exec {A[1]}>&- both work,
and a subscript may be a variable ({A[i]}).

Why

Follow-up to #2605. That PR allowed brackets in the name but the check still
required the word to be a single literal part, and {A[1]} lexes as several —
[ opens a glob bracket. So the redirect parsed as a command:

$ exec {A[1]}> o.txt
bash: {A[1]}: command not found    # and status 127

Before / After

declare -a A
A[1]=7
exec {A[1]}> o.txt
echo hi >&"${A[1]}"
exec {A[1]}>&-
cat o.txt

	output	status
bash 5.2	hi	0
bashkit before	bash: {A[1]}: command not found	127
bashkit after	hi	0

A brace word that is not an fd variable is still a command: {A-1}> o.txt
reports command not found in both shells.

Risk

Low and parser-local. The name is now read from the word's literal parts
joined; a word with any non-literal part (an expansion, a substitution) yields
nothing and is left alone, so ${x}> file and $(cmd)> file parse as before.

Checklist

• cargo test -p bashkit --lib (2920 passed)
• cargo test -p bashkit --test integration (1372 passed)
• New spec case 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)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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