[codex] Expose basic PGN import API

[NSStudent]

Summary

Expose the lightweight BasicPGN import path so external consumers can parse PGN metadata and raw movetext efficiently.

Changes

• Make BasicPGNParser public and add BasicPGNReader as the simple public entry point.
• Add Equatable, Sendable, Codable, and convenience tag accessors to BasicPGNGame.
• Preserve custom tags explicitly and support escaped PGN tag values in TagParser.
• Add public API tests without @testable import covering custom tags, escaped values, multi-game parsing, and Codable round-tripping.

Validation

• rtk swift build
• rtk swift test --filter BasicPGNPublicAPITests
• rtk swift test --filter PGN
• rtk swift test

[codecov]

Codecov Report

❌ Patch coverage is 91.34615% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Sources/FischerCore/PGN/Parsers/TagParser.swift	86.95%	9 Missing ⚠️

Files with missing lines	Coverage Δ
Sources/FischerCore/PGN/Model/BasicPGNGame.swift	100.00% <100.00%> (ø)
...urces/FischerCore/PGN/Parsers/BasicPGNParser.swift	100.00% <100.00%> (ø)
Sources/FischerCore/PGN/Parsers/TagParser.swift	86.95% <86.95%> (-13.05%)	⬇️

... and 1 file with indirect coverage changes

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