Fix overflow when a parameter range ends at the numeric type's maximum

[santhreal]

Problem

RangeStep::next yields the current value and then unconditionally computes state + step:

let return_val = self.state;
self.state += self.step;
Some(return_val)

When end is at (or within step of) the numeric type's maximum, that final increment overflows. RangeStep::new allows such a range through as long as it's small enough (the size check passes), e.g. --parameter-scan x 2147483645 2147483647:

• debug builds: panics with attempt to add with overflow
• release builds: state wraps around (to i32::MIN), so state > end stays false and the iterator runs far past end instead of terminating.

Fix

Track a done flag and only advance state while state + step stays within end; otherwise the current value is the last element. end - state is non-negative (state <= end is already checked) and step is positive (validated in new), so the guard itself can't overflow.

Tests

Adds test_range_reaching_type_max_terminates (a range ending at i32::MAX), which panics on the old code and passes now. Existing range_step tests are unchanged and still pass; cargo fmt --check is clean.