fix(interpreter): resolve nameref for ${!ref[@]} key enumeration

[chaliy]

Summary

• ${!ref[@]} now correctly returns keys when ref is a nameref to an associative array
• The ArrayIndices handler in expand_word_to_string was not resolving namerefs before looking up keys
• The expand_word_to_fields path already had the fix; this aligns the string expansion path

Test plan

• nameref_assoc_key_enumeration_string — verifies ${!ref[@]} returns keys through nameref
• Full spec test suite green
• Clippy clean

Closes #846