fix(interpreter): handle ++/-- in complex arithmetic expressions

[chaliy]

Summary

• Fix (( ++i > N )) && break exiting while loops on the first iteration
• execute_arithmetic_with_side_effects now extracts the variable name from ++var/--var, performs the side effect, and evaluates the remaining expression with the new value substituted
• Added spec tests for both pre-increment and pre-decrement in loop break patterns

Test plan

• cargo test --test spec_tests -- bash_spec_tests passes (1829 specs, 0 failures)
• New tests arith_preincrement_and_break and arith_predecrement_and_break verify correct loop iteration count
• cargo clippy -- -D warnings clean
• cargo fmt --check clean

Closes #903