Preserve PGN tags on load -> save

[BrandonZacharie]

Summary

Loading a PGN, saving it back, and re-loading the saved file used to lose everything except the moves — Event / Site / Date / Round / White / Black / Result all reset to the engine's defaults. For anyone loading a famous game and saving it back, that's a surprise. This PR preserves the metadata across a load → save round-trip.

Changes

• Game.pgn_tags: Dict[str, str] — new public field, defaults to {} on a fresh game; copy semantics in the from-existing-game ctor branch.
• PGNFile.game() — populates pgn_tags from the pgnparser.PGNGame attributes for the Seven Tag Roster plus Annotator, PlyCount, TimeControl, Time, Termination, and Mode. Only non-empty values are stored.
• Game.save_pgn() — tags now layer as built-in defaults < self.pgn_tags < caller headers=. So:
  • Load → save reproduces the original metadata.
  • A fresh game still uses the engine's defaults (Casual Game / Chess.py vX.Y.Z / today / ? / ? / ? / derived Result / counted PlyCount).
  • A caller passing headers= can still override anything (used by callers, never invoked from the UI today, but documented behavior).

The PGN-recorded Result wins over the derived one when both are present — important for games that ended by resignation or agreement (and never reached an on-board mate), where game.result() would otherwise say *. For a brand-new game played to mate, the derived Result still gets used (no pgn_tags entry to override it).

Scope notes

JSON save/load is intentionally untouched. pgn_tags lives on the Game object but isn't serialized to JSON, so a PGN → JSON → PGN cycle still loses the tags. Threading pgn_tags through serializable() / parse_json_file() is the natural follow-up but adds a JSON format change and tests on the JSON path, so it didn't make sense to bundle here.

Tests

Five new tests in tests/test_game.py:

• Fresh Game() has pgn_tags == {}.
• PGNFile.game(0) against test1.pgn populates all seven STR tags correctly.
• Load → save reproduces every STR tag including the recorded Result "1/2-1/2" (the Fischer–Spassky draw — note game.result() would say * since no mate occurred).
• Caller headers= overrides both defaults and loaded tags.
• A brand-new Game still emits the Casual Game / ? / ? defaults.

Test plan

• make test — fast suite (213 passed, 2 deselected) in ~1.4 s.
• make test-all — full sweep including Carlsen/Nakamura passes in ~7:42. Verified locally.
• coverage report after make test shows TOTAL 100%.
• Manual: load tests/test1.pgn, save as PGN to a new path, diff the two — Event/Site/Date/White/Black/Result/Round all match the input.
• Manual: start a new game, save as PGN — Event is Casual Game, White/Black are ?.

https://claude.ai/code/session_01RnQvxtXfn9JFTN8jpNyriL

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