fix(redir): validate the fd word of a {var} dup redirect

[brianjfox]

Summary

{v}<&word / {v}>&word converted the target word with atoi, so a non-numeric word (exec {v}<&foo) silently duplicated fd 0 and succeeded. bash rejects any word that is not a plain fd number as an ambiguous redirect naming the variable (v: ambiguous redirect), leaves the variable untouched, and fails the redirection with status 1.

Closes #656

Changes

• core/src/executor.cpp (dup path): validate the word before converting — foo, 0junk, -1, +1, and empty expansions are v: ambiguous redirect. Conversion uses strtoll with a range check: an fd beyond INT_MAX cannot exist and reports Bad file descriptor instead of being truncated into a valid fd. A failed dup prints bash's two-line report for the {var} form — the generic cannot duplicate fd line, then the converted number (010 reports as 10), or the raw digits when the value doesn't fit an intmax. Two bash-side quirks (it prints an uninitialized garbage number for <&"" and for set-variable non-move targets — nondeterministic across runs) are deliberately not replicated; gnash reports deterministically.
• core/src/executor.cpp (separate commit): the issue's repro (set -e; exec {v}<&foo; echo success) exposed that a failed redirection on a simple command returned status 1 but skipped the ERR trap and set -e entirely — a pre-existing gap affecting every redirect failure (set -e; echo hi > /nonexistent-dir/f; ... also kept going). It now mirrors the compound-command path, including ! inverting the failure to success.
• tests/harness/run_diff.sh: 14 differential cases (484 total).

Verification

• The issue's exact repro now matches bash byte-for-byte: v: ambiguous redirect, exit 1, no success.
• 26-case probe matrix vs bash 5.3 matches on stdout and exit status (ambiguous forms, valid dups and moves, converted-number failure reports, overflow, ERR trap, set -e, ! inversion, errexit-suppressed contexts).
• run_diff 484/484 (three consecutive clean runs); ctest 23/23; full 83-suite scoreboard sweep with fresh oracles: no regressions (redir/vredir 0; only the standing comsub-eof=5 and the pre-existing errors=125 of errors.tests: remaining fatal-error divergences (readonly/posix aborts, ${$var} and ${-N} substitutions, set -u statuses) #658 remain).