fix(interpreter): named file descriptors ({var}>file) and <> like bash

[chaliy]

Requested by Михайло · project thread

What changed

Named file descriptors now behave like bash 5.2:

• {var} works with every redirect form (>, <, >>, <>, <<<, <<, >&N, <&N) on any simple or compound command, and the fd stays open afterwards.
• Allocation takes the lowest free fd from 10 up and ignores the variable's old value. Locals and array elements work; a readonly variable fails.
• {var}>&- closes the fd, and a closed fd is reused.
• >&N / <&N to a closed fd prints Bad file descriptor with status 1; a dup to a word prints ambiguous redirect.
• The lexer accepts multi-digit fds (10>x), <> and N<<<word, and declare -f prints {var} redirects back.
• exec applies its redirects in order and no longer applies them a second time, which fixes exec 1>&3 3>&-.

Fds stay virtual and per interpreter; no host fds are touched.

Why

BashBox corpus batch: all 17 skipped cases in bashbox-named-fd.test.sh now pass, plus readwrite_redirect_opens_file (<> used to be a parse error).

Before / After

exec {fd}>f; echo $fd; echo hi >&$fd; exec {fd}>&-; cat f; echo x {o}>g; echo $o; echo hi >&9; echo rc=$?

before: "" / "hi" / "" / rc=0       # no fd allocated, x went to g, >&9 silently succeeded
after:  10 / hi / x / 10 / bash: 9: Bad file descriptor / rc=1   # matches bash 5.2

Risk

• Medium: the redirect path is shared with exec.
• TM-DOS-063 tests were updated: bash ignores a pre-set v=-1 and allocates fd 10, so the old tests expecting "invalid file descriptor" were replaced. The fd cap (max_file_descriptors) still applies and has a new test.
• Remaining gaps are pre-existing and recorded in limitations.md (no shared position for N<>file, 3<file also feeds stdin, output to fds 3-9 from a command's own redirect is dropped).

Checklist

• Tests added or updated
• Backward compatibility considered

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