[repo-assist] feat: add TimeSpan interop — Add(TimeSpan), operator+ and operator-

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Summary

Add natural .NET TimeSpan interoperability to DateTimeNano:

• DateTimeNano.Add(TimeSpan) — shift a timestamp by a TimeSpan
• operator +(DateTimeNano, TimeSpan) — shift forward
• operator -(DateTimeNano, TimeSpan) — shift back

Rationale

System.DateTime supports +/- with TimeSpan out of the box, and many .NET APIs return durations as TimeSpan. Adding the same operators to DateTimeNano makes the two types easier to use together:

Before:

var later = timestamp.AddMilliseconds((long)myTimeSpan.TotalMilliseconds);
var earlier = timestamp.AddNanoseconds(-myTimeSpan.Ticks * 100);

After:

var later   = timestamp + myTimeSpan;
var earlier = timestamp - myTimeSpan;

The operators complement the existing operator -(DateTimeNano, DateTimeNano) → long and the pending operator +(DateTimeNano, long) / operator -(DateTimeNano, long) in PR #15.

Implementation notes

• Delegates to the existing AddNanoseconds(long) (TimeSpan.Ticks * 100 converts ticks → nanoseconds). Overflow/underflow protection is fully inherited.
• TimeSpan precision is 100 ns (one tick); sub-tick nanoseconds stored in DateTimeNano are unaffected.
• No new dependencies. No breaking changes.

Test Status

✅ 44 tests pass on net8.0 (40 original + 4 new tests covering Add(TimeSpan) and both operators for positive and negative durations). Build succeeded for net8.0, net9.0, and net10.0 with 0 warnings.

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