test(interpreter): direct test for non-global Function closure isolation

[humancto]

Background

PR #114 added 5 unit tests for Environment::deep_clone_isolated. All five exercise either:

• Value::Lambda (which always goes through the full closure path), or
• top-level fn (which hits the is_global_fn fast path in call_function_inner and bypasses the closure entirely).

The interesting middle case -- a named inner function returned from another function -- produces a Value::Function with closure.scopes.len() > 1 and does exercise the deep_clone_isolated Function branch. Currently untested.

What to add

Direct test:

#[test]
fn fork_for_serving_isolates_non_global_function_closure() {
    let mut template = Interpreter::new();
    template.run_repl(...
        // fn outer() { let mut x = 0; fn inner() { x = x + 1 }; return inner }
        // let bump = outer()
    ...).expect("setup");

    let mut a = template.fork_for_serving();
    let mut b = template.fork_for_serving();

    // bump in fork a is a Value::Function with closure.scopes.len() > 1
    // calling it should mutate fork a's `x`, not template's, not b's
}

Branch is implemented and almost certainly correct (parallel to Lambda) but the gap should be closed.

Origin: rust-expert review on PR #114, follow-up #2.