Signed integer overflow in scan_arbitrary.hpp

Undefined behaviour sanitizer on Linux with Clang complains about this:

When "Unit" is int, a call to validate_scan in Line 148 causes a "std::numeric_limits::min -1" in line 174

Start in scan_arbitrary.hpp at validate_scan around lines 148 and 174, and reproduce the UBSan report with Clang on Linux. Trace the signed-integer minimum-value calculation and verify that the validation no longer triggers undefined behavior while retaining its intended checks.

Written by the indexing model from the issue text.