test: add Game current position invariant coverage

[CorentinGS]

Summary

• Adds test-only Game current-position invariant coverage for construction, move application, navigation, minimal PGN parsing, cloning, and splitting.
• Fixes Game.Clone() to preserve the current cursor and keep Position(), CurrentPosition(), g.pos, and currentMove.position consistent.
• Completes the first implementation slice from local Issue 002: Deepen the Game Module.

Verification

• go test ./...

[Copilot]

Pull request overview

This PR improves Game.Clone() correctness by preserving the game’s current move cursor (and corresponding current position) when cloning, and adds invariant-focused tests to ensure Game consistently reports its current position across common operations.

Changes:

• Update Game.Clone() to map g.currentMove into the cloned move tree (instead of defaulting to the last mainline move).
• Ensure cloned games’ pos tracks the cloned currentMove.position.
• Add invariant tests covering current-position consistency after cloning, navigation, PGN parsing, and Split().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
game.go	Fixes clone behavior by preserving the current move cursor via a move-tree search and syncing pos to the cloned cursor position.
game_invariants_test.go	Adds tests asserting Game’s “current position” invariants across clone/navigation/PGN/split workflows.

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