TypedPython: an ArrayParam function that calls user code compiles and diverges from CPython #203

Defect (identity). The ArrayParam contract (ir.py) says a function holding a list copy-in runs no user code, but neither the front end nor the verifier enforces it.

Reproduction on develop: def f(xs: list[float], cb: Callable[[], None]) -> float: cb(); return xs[0] compiles. With cb setting xs[0] = 100.0, compiled f returns 1.0 (it read its copy) while the interpreter returns 100.0. A store into the array followed by a callback is also overwritten by write-back.

Fix: the verifier rejects (the function stays interpreted with a reason) a function with an ArrayParam that is not closed. Not closed means it runs GetAttr, CallObject, Truth, CompareObj, ObjToFloat or OBJ BinOp, or calls a function that runs user code (the existing closedness fixpoint), or uses FieldGet/FieldSet/New, whose slow paths run user code after a class change.

Done when: the reproduction stays interpreted and equals CPython, there are IR-level verifier tests for each open node and for the transitive case, a mutation that drops the rule makes the end-to-end test fail, and the suite is green.