Handle duplicate PGN tags

[NSStudent]

Summary

• Allow PGN tag parsing to accept duplicate tags by keeping the last value for a repeated key.
• Add a tournament PGN regression case with duplicate FEN tags.

Root Cause

TagParser built its metadata dictionary with Dictionary.init, which traps when the parsed tag list contains duplicate keys. Some tournament PGNs can include two FEN tags, causing parsing to crash before the game is returned.

Validation

• swift test --filter PGNGameParserTests
• swift test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
Sources/FischerCore/PGN/Parsers/TagParser.swift	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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