feat: implement Game.MoveHistory() method

[CorentinGS]

Summary

• Add MoveHistory struct with PrePosition, PostPosition, Move, and Comments fields
• Add Game.MoveHistory() method traversing the main-line move tree
• Add 7 tests covering empty games, main-line traversal, comments, variations exclusion, length consistency, and PGN integration
  Closes MoveHistory is not defined #78

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.18%. Comparing base (28504d8) to head (f1b1116).

Files with missing lines	Patch %	Lines
game.go	90.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   70.09%   70.18%   +0.09%     
==========================================
  Files          27       27              
  Lines        4146     4166      +20     
==========================================
+ Hits         2906     2924      +18     
- Misses       1109     1110       +1     
- Partials      131      132       +1     

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[Copilot]

Pull request overview

Implements a new Game.MoveHistory() API to expose main-line moves together with their pre-/post-move positions and any attached move comments, addressing the missing functionality referenced in #78.

Changes:

• Added exported MoveHistory struct (pre-position, post-position, move, comments).
• Added (*Game).MoveHistory() method that traverses the main line (first-child path), excluding variations.
• Added unit tests covering empty games, main-line traversal, comment presence/absence, variation exclusion, length consistency vs Moves(), and PGN-loaded games.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
game.go	Adds MoveHistory struct and Game.MoveHistory() main-line traversal implementation.
game_test.go	Adds tests validating MoveHistory() behavior across several scenarios including PGN integration.

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