test: add complex Date edge case tests for DeepStrictMerge

[kakasoo]

Summary

• Closes DeepStrictMerge does not handle Date types, causing Date decomposition #38
• The Date handling fix was already present in DeepStrictMerge.Infer (lines 18-22), so no source change was needed
• Added 5 new complex edge case tests to verify Date behavior in more challenging scenarios:
  • Target Date vs Source non-Date object at the same key
  • Target non-Date object vs Source Date at the same key
  • Date properties inside array element merging
  • Date mixed with deeply nested objects (3+ levels)
  • Multiple Date properties at various nesting levels

Test plan

• npm run build:test && npm run test — all 390 tests pass (5 new)
• npm run prettier — formatting applied

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