fix(interpreter): special params $0, $-, BASH_SUBSHELL, positional slices

[chaliy]

Requested by Михайло · project thread

What changed

Special parameters now behave like bash:

• $0 keeps the script name inside functions and sourced files.
• $- lists the active short options (h and B always on).
• $BASH_SUBSHELL tracks the depth of ( ), $( ) and compound pipeline stages.
• ${@:off[:len]} / ${*:off} slice positional parameters, with $0 at offset 0.
• ${#} and ${-…}, ${?…}, ${$…} with operators now parse.
• read -a keeps empty fields between delimiters.

Why

These are level-up roadmap step 1 fidelity gaps from the Debian-oracle shell cases. Scripts use $0 for usage messages, case $- in *i*) for interactivity checks, and "${@:2}" for argument forwarding.

Before / After

Script	Before	After (= bash)
f(){ echo $0; }; f	f	script name
echo "${-//c/}"	``	hB
echo $BASH_SUBSHELL $(echo $BASH_SUBSHELL) $( (echo $BASH_SUBSHELL) )		0 1 2
set -- a b c d; echo ${@:1}	b c	a b c d
set -- a b c d; echo ${#}	0	4
IFS=, read -ra p <<< a,b,,d; echo ${#p[@]}	3	4

Proof: new spec special-vars-subshell.test.sh (bash-verified), bash_comparison passes.

Risk

Low. $- now reports hB, and one security test was updated for that. BASH_SUBSHELL is a counter that is saved and restored with the subshell snapshot.

Checklist

• Failing spec first, then fix
• cargo test -p bashkit --lib / --test integration / bash_comparison
• clippy -D warnings, fmt
• knowledge/log.md updated

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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