fix: handle Date as leaf type in DeepStrictMerge

[kakasoo]

Summary

• Add Date type guard in DeepStrictMerge.Infer to prevent recursive merging into Date's internal properties
• Date extends object is true, so without the guard, Date properties were being destructured during deep merge
• Consistent with how DeepStrictOmit, DeepStrictUnbrand, and DeepDateToString already handle Date as a non-recursive leaf

Test plan

• Added test_types_deep_strict_merge_date_preserved — disjoint Date properties from Target and Source
• Added test_types_deep_strict_merge_overlapping_date — same Date key in both Target and Source
• Added test_types_deep_strict_merge_nested_date — Date properties inside nested objects
• All 379 tests pass (no regressions)

Closes #20

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