parser, executor: implement named file-descriptor redirections ({var}>file)

[brianjfox]

Fixes #275.

Root cause

The {varname} file-descriptor redirection form was unsupported — {fd} was taken as an ordinary word (exec {fd}>file → exec: {fd}: not found).

Implementation

• AST: Redirect.fd_var.
• Parser: recognize an unquoted {name} glued to a redirection operator (in both parse_simple and parse_redirect_list), so a {name} before a redirection is the fd-variable specifier rather than a command word.
• Executor: apply_redirect allocates a fresh high, close-on-exec descriptor, opens the redirection on it, and stores its number in the variable. It is recorded as an originally-closed fd, so a normal command closes it afterwards while exec keeps it open (the variable keeps the number). {var}>&-/{var}<&- close the descriptor the variable currently names.
• to_string: declare -f reconstruction emits the {name} prefix.

Supports <, >, >>, <>, >&N, <&N, >&-, <&-, and <<EOF, on simple and compound commands, with exec (permanent) or a command (temporary).

Verification

• exec {fd}>file; echo hi >&$fd; exec {fd}>&-; cat file → hi; {fd}<file reads; {fd}<>file, {fd}>&1, heredoc {v}<<EOF, group { ...; } {fd}<file — all match bash 5.3; the variable holds an integer ≥ 10.
• Non-glued {fd} >file, quoted "{fd}", and brace expansion {a,b}>file are unaffected.
• vredir scoreboard: 125 → 33.
• Execution differential 234/234; real-script parser corpus 429/429; negative corpus 68/68 (0 leniency).

Known limitations (the residual vredir diffs)

• bash's exact cannot assign fd to variable / ambiguous redirect / Bad file descriptor error text for unassignable (nameref/readonly) or invalid fd-vars is not reproduced; because those cases now run instead of failing to parse, nameref ticks up slightly (248 → 258) — a byte-diff artifact, not broken behavior.
• Array-element fd targets ({fd[0]}<&0) and bash's specific high-descriptor numbering are not implemented.