TypedPython: chunked range loops poll every 16.7M iterations instead of every 4096 #280

cgen.py ~2081 (#206 chunked polling) runs a chunk of TP_POLL_INTERVAL iterations and then calls self.poll(), which emits the countdown form tp_pc_tick(&tp_pc) && tp_poll_slow(...). The countdown also starts at TP_POLL_INTERVAL, so tp_poll_slow runs once per 4096*4096 iterations. A compiled loop under ~16.7M iterations never reaches a safepoint: it blocks stop-the-world (gc.get_objects on 3.15t), KeyboardInterrupt and other eval-breaker work for its whole duration. Found while writing the #143 gc race test.

Done when: the chunk loop calls the slow poll once per chunk; a test asserts a signal/breaker set during a long compiled loop is seen within about TP_POLL_INTERVAL iterations.