fix(builtins): printf -v, readonly assignment, quoted $@ fields and */ globs like bash

[chaliy]

Requested by Михайло · project thread

What changed

Closes 16 of the 17 skipped BashBox declare / just-bash-compat spec cases, plus printf invalid-number errors.

• printf -v parses options like bash (-vNAME, repeated -v, --); a missing -v argument or format, a bad option or an invalid name fail with status 2 and the usage line.
• printf -v assigns through the interpreter like read: declare -i targets evaluate, a[i] sets elements, locals stay local.
• read, printf -v and getopts into a readonly variable fail with readonly variable, status 1 (silently succeeded before).
• Quoted $@ inside a larger word gives one field per element ("x$@y", x"${a[@]}"y); ${1+"$@"} keeps the original fields.
• A glob ending in / matches only directories (and symlinks to them) and keeps the slash.
• cp -n no longer overwrites.
• $'\xc3\xa9' decodes as UTF-8 text.
• printf '%d' abc / 12abc print 0 / 12, report printf: abc: invalid number, exit 1.

Why

Bash-verified cases from the BashBox corpus import (#2585) that bashkit failed.

Before

$ set -- a b; printf "[%s]" "x$@y"; echo
[xa by]
$ mkdir d1 d2; touch f1; echo */
d1 d2 f1
$ readonly r; printf -v r %s a; echo $?
0
$ printf -vy %s a; echo $y
-vy
$ printf '%d\n' abc; echo rc=$?
0
rc=0

After

[xa][by]
d1/ d2/
bash: line 1: r: readonly variable
1
a
0
bash: line 1: printf: abc: invalid number
rc=1

Matches real bash (bash_comparison_tests pass).

Risk

Low to medium: printf option parsing and quoted-$@ field splitting are widely used; covered by existing and new spec cases plus real-bash differential tests. One case stays skipped (declare -p of $'\xff', L-STREAM-001).

Checklist

• cargo fmt --check, clippy -D warnings
• cargo test -p bashkit -p bashkit-cli --lib --bins --tests -- --include-ignored
• Knowledge updated (foundations/builtins.md, operations/limitations.md, log.md)

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