fix(interpreter): resolve namerefs before nounset check

[chaliy]

Summary

• Nameref variables are now followed to their target before checking if a variable is unbound under set -u
• Previously, the nameref itself was checked, causing false "unbound variable" errors even when the target was defined

Test plan

• nameref_nounset_basic — basic nameref under set -u
• nameref_nounset_local_n — local -n inside function under nounset
• nameref_nounset_array — nameref to array under nounset
• nameref_nounset_write_through — nameref write-through under nounset
• nameref_nounset_harness_pattern — set -euo pipefail with nameref function
• All 1753 bash spec tests pass
• No regressions

Closes #834