fix(interpreter): apply redirects left-to-right for correct fd duplication

[chaliy]

Summary

• Redirects are now applied left-to-right so 2>&1 >file correctly captures stderr in $() while writing stdout to file
• Previously both stdout and stderr went to the file; now fd duplication respects the state at the time each redirect is processed

Test plan

• New redirect ordering spec tests
• Full spec test suite green

Closes #853