fix(interpreter): ERR/RETURN trap scoping and N>&word redirects like bash

[chaliy]

Requested by Михайло · project thread

What changed

• ERR trap scoping follows bash. ERR fires right after the failing command (simple commands, function calls, eval, multi-command pipelines, ( ), [[ ]], (( ))); lists and compound commands no longer fire it a second time. Functions, subshells, $( ) and pipeline stages don't inherit ERR unless set -E is on; a trap set inside a function stays set; a function that sets ERR doesn't fire it for its own call. $LINENO in the handler is the failing line.
• RETURN trap runs. It fires when a function or sourced file finishes, keeps $?, and is inherited by functions only under set -T.
• N>&word redirects. 2>&- closes stderr for that command (it used to act like 2>&1), >&file / 1>&file mean &>file, 2>&file is an ambiguous redirect, and 2>&$fd duplicates the fd instead of writing a file named after it. A failed &> open no longer prints the command's output.
• for ((...)) re-runs $(...) in its init, condition and step.

Why

BashBox corpus batch: all 17 skipped cases in bashbox-interpreter-builtins.test.sh and all 10 in bashbox-err-trap.test.sh now pass (10 of the 27 were already fixed by earlier batches and only lost their skip lines).

Before / After

trap "echo err" ERR; f(){ false; echo in; }; f; x=$(false; echo sub); echo "[$x]"; ls /nope 2>&-; { echo z; echo e >&2; } >& both.txt; cat both.txt

	Output
Before	err in / [err sub] / ls: cannot access '/nope': ... / z (stderr lost)
After (= bash 5.2)	in / [sub] / err / z / e

set -T; trap 'echo r' RETURN; g(){ :; }; f(){ g; }; f; echo end printed only end; it now prints r r end, like bash.

Risk

• Medium: ERR firing moved from list/script level into execute_command; covered by 9 new trap-scope cases plus the corpus, all recorded from bash 5.2. New handling sits in boxed helpers so the recursion stack budget (function_recursion_is_bounded_on_a_two_mib_stack) is unchanged.
• Known gaps (in limitations.md): a write error after a command's own >&- doesn't name the command; the DEBUG trap still runs in subshells without set -T.

Checklist

• Tests added or updated
• Backward compatibility considered

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