CombineFrontmatter silently removes repeated items from every list, even when no keys are being merged #142

What's wrong

PropertyMerger groups every key, including singleton groups (PropertyMerger.cs:47-52), and runs each group through MergePropertyGroup. Any list value is routed to MergeArrayValues (line 158). That method collects items into a HashSet<object> (line 175), which drops duplicates within a single list, not just duplicates across the lists being merged.

Failure scenario

Input document, one frontmatter block, no colliding keys:

---
steps: [mix, wait, mix]
scores: [1, 1, 2]
---

With CombineFrontmatter under the default (Conservative) strategy, the output is:

steps: [mix, wait]
scores: [1, 2]

With FrontmatterMergeStrategy.None, both lists are preserved. Ordered lists that can legitimately repeat values (recipe steps, scores, playlists, repeated authors/contributors, and so on) lose data just from normalizing a single-block document. Nothing is being merged, so the user gets no warning.

Suggested fix / acceptance criteria

• Pass single-key groups through unchanged, since there is nothing to merge.
• When several keys really are merged, append the lists in order. Drop an item only if it already came from a different source list (or don't dedupe at all). Never dedupe within one list.
• Tests:
  • A singleton steps: [mix, wait, mix] round-trips unchanged through CombineFrontmatter.
  • Merging tags: [a, a] with tag: [b] keeps both a entries.