TypedPython gate: detect type-changing rebinding and mixed-element containers #22

What

The two §3.1 forbidden items the gate does not catch yet (SPEC N-6): x = 1 followed by x = "a", and [1, "a"] inferred as list[int | str]. Pyrefly 1.3.2 reports neither, and its type report has no entries for assignment targets. Find a source for the inferred types of assignment targets, for example Pyrefly's infer output or reading the stored names back through a load, and report both: warnings in checked, errors in compiled.

Done when

Tests for both cases go red, then green. An explicitly annotated union (list[int | str]) is not flagged.