TypedPython: verifier and cgen disagree on whether an F64/BOOL Unbox can deopt #271

Found during #143. verify.py e_Unbox sets can_deopt only for an I64 Unbox (and test_verify.py asserts an F64 Unbox is fine in an impure function), but cgen emits tp_unbox_f64 / tp_unbox_bool, which return 1 (deopt) on a non-exact operand, and cgen.py treats every Unbox as may-deopt. In an impure function a deopt after an effect would repeat that effect on redo. The front end currently emits no Unbox, so source programs do not reach it; hand-built IR does.

Done when: verifier and cgen agree (either every scalar Unbox can deopt, or F64/BOOL Unbox is only accepted on an operand proven exact, with cgen emitting no deopt branch), and a test pins the agreed rule.