Fix overflow/underflow detection in join_seconds

[derekmauro]

Improve the precision of overflow and underflow checks in the join_seconds template function.

• Use int64_t for intermediate subsecond calculations to prevent premature overflow when the target representation type is small.
• Refine the underflow check to correctly handle boundary cases near the minimum limit, avoiding unsafe subtraction.
• Handle cases where the duration denominator is larger than the maximum limit of the representation type.

Fixes #199