TypedPython: interval proofs drop provably safe integer checks #148

Approved by the user (2026-10-04). Checked i64 arithmetic (overflow, then deopt) stays on every op today. Extend the verifier's interval proofs: e.g. i + j with both below len(list) cannot overflow; floor division by a power of two of a proven non-negative value becomes a shift. Unprovable checks stay. No identity cost.

Done when: the verifier re-proves every dropped check, a mutation that drops an unproven check is rejected by the verifier, and spectral-norm/fannkuch are re-measured.