refactor(interpreter): extract LoopAccumulator for errexit suppression tracking

[chaliy]

Summary

• Extract duplicated loop result accumulation into LoopAccumulator helper in state.rs
• Consolidates stdout/stderr/exit_code/errexit_suppressed tracking + break/continue/return propagation
• Used by execute_for, execute_arithmetic_for, and execute_condition_loop
• Net: +206 / -175 lines (adds helper + 6 unit tests, removes 3x duplicated control flow blocks)

Test plan

• All existing tests pass (cargo test --all-features)
• 6 new unit tests for LoopAccumulator (accumulate, break, continue, return, finish)
• cargo clippy -- -D warnings clean
• cargo fmt --check clean

Closes #881