Merge duplicate token analyses on PT9 import (#313)

[alex-rawlings-yyc]

Closes #313.

Summary

• PT9 import merges token analyses with identical content into one, the same way hand-glossing already does. This uses the analysesAreIdentical rule, so surface forms differing only in case also merge. It covers both analyses from interlinear clusters and the unlinked ones from the word-analysis inventories.
• Every link to a dropped duplicate moves to the analysis that is kept, and keeps its own token's surface text, so drift detection stays per token.
• If merging leaves one token with two links to the same analysis, only one is kept: the approved one if there is one, otherwise the first.
• The import report has a new merge.identicalPayloadsMerged count. The import modal doesn't show it.

buildBareWordAnalyses keeps its own duplicate check, so skippedExistingIdentical still counts what it did. The merge runs after every other conversion step, so analyses made identical by #324's blank-morpheme stripping will merge too.

Out of scope: a merged analysis shows the surface text of whichever token came first, e.g. a capitalized sentence-initial form; deriving the displayed form from its links belongs to #186. PT9 import still writes one phrase payload per occurrence; that has its own issue.

Test plan

• Unit tests for merging duplicates and moving their links, an unlinked analysis matching a linked one, case variants, the three near-misses that must stay apart (different gloss, breakdown vs. none, different breakdown), and the one-link-per-token rule; 100% coverage
• Import PIA (test-data/pt9-projects/PIA): the four plovs tokens in PHP 1:6-7 end up on three analyses, with the two "catorce / plov s" occurrences sharing one
• The catalog shows that analysis as one row with two uses, and the plovs suggestion dropdown has no duplicate rows

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