fix(interpreter): short-circuit && and || inside [[ ]] for set -u

[chaliy]

Summary

• Add evaluate_conditional_words that operates on raw Word tokens with lazy expansion
• && short-circuits: if left is false, right side is never expanded
• || short-circuits: if left is true, right side is never expanded
• Prevents spurious "unbound variable" errors under set -u

Test plan

• Spec tests: 4 cases covering &&/|| short-circuit with set -u
• Full test suite passes (2220 tests)

Closes #939