Fix LT-21598: Find and Fix repairs LexReferences with duplicate Targets

[mark-sil]

Summary

A LexReference (lexical relation) could list the same Target object more than
once. This invalid state can lead to crashes — the incoming-reference index
deduplicates, so the duplicated Target and the relation's references get out of
sync, leaving stale references to objects when they are later processed.

This is the data-repair companion to the runtime fix in #380: that PR makes the
model robust against the corrupt data, and this PR teaches Find and Fix to
repair it.

Change (SequenceFixer)

Find and Fix now counts distinct Targets of a LexReference:

• If duplicates exist but two or more distinct Targets remain, the duplicate
  Targets are removed (the first occurrence of each is kept).
• If fewer than two distinct Targets remain, the whole LexReference is
  removed (and the owning LexRefType's reference to it). This generalizes the
  existing "too few Targets" rule, which previously counted raw entries and so
  missed e.g. a relation listing one sense twice.

Testing

• New unit test DuplicateLexReferenceTargets covering the three cases:
  duplicate removed (relation kept), all-duplicate (relation removed), and a
  valid relation left untouched. Existing fixer tests still pass.
• Validated end-to-end on the project from the bug report: the actual
  FwDataFixer made 42 repairs (38 duplicate Targets removed, 4 now-invalid
  relations removed), leaving 0 LexReferences with duplicate or fewer-than-two
  distinct Targets.

🤖 Generated with Claude Code

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This change is 

[github-actions]

LCM Tests

    16 files  ±0      16 suites  ±0   2m 48s ⏱️ -27s
 2 861 tests +1   2 841 ✅ +1   20 💤 ±0  0 ❌ ±0 
11 392 runs  +4  11 224 ✅ +4  168 💤 ±0  0 ❌ ±0 

Results for commit 4fada45. ± Comparison against base commit c4d88bb.

[jasonleenaylor]

@jasonleenaylor reviewed 6 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on mark-sil).