Key the combinations name cache by reference

[SimonCropp]

CombinationResultsConverter caches the rendered name per key in a Dictionary<object, string>, which used the boxed value's own equality. Two keys that compare equal but render differently therefore shared one name:

• DateTime.Equals ignores Kind, but the name carries it.
• DateTimeOffset.Equals compares only the instant, but the name carries the offset.

A combination over 2000-01-01 as Utc, Local and Unspecified labeled all three rows 2000-01-01Utc — so the output had duplicate property names and no way to tell the rows apart. Same for 2000-01-01T00:00+00:00 against 2000-01-01T01:00+01:00.

Each input list is materialized once (a.Cast<object?>() then ToArray() in the CombinationRunner constructor) and rows index into that array, so a key recurs as the same boxed instance. Reference equality is what the cache actually wants, and it sidesteps the value equality traps entirely — that is now what it uses, via a small internal ReferenceComparer.

Worth noting Counter already treats these as distinct (DateTimeComparer compares Kind, and there is a DateTimeOffset equivalent), so this brings the converter in line with the rest of the codebase.

Test CombinationTests.DateKeysThatCompareEqual combines the three DateTimeKind values against two equal-instant offsets. On main all six rows come out labeled 2000-01-01Utc, 2000-01-01+0; with the fix each row is distinct.

Verify.Tests (1300) and StaticSettingsTests pass, and Verify builds for all nine target frameworks.