Fix restoring off-hand and cursor items after games

[GerwinVerkerk]

Describe your changes

Preserve the player's extra inventory contents and cursor item in the existing pre-game PlayerData snapshot.

PlayerInventory#getExtraContents() includes the off-hand slot on Paper. HungerGames previously cleared that slot when a game started but only restored storage and armor contents, causing off-hand items to disappear after leaving or finishing a game.

This change:

• snapshots all extra inventory slots before clearing the player;
• snapshots and clears the cursor item so it cannot enter the arena;
• restores both extra contents and the cursor alongside the existing storage and armor contents;
• keeps the existing game, join, leave, and restore flow unchanged.

This intentionally does not address the large-arena preparation issue or add durable crash recovery; those have different risk and review scope. A follow-up proposal for durable snapshots is described on issue #113. It also does not duplicate the world-border work in #115.

Verification

• ./gradlew clean build --no-daemon
• git diff --check
• Build completed successfully against the repository's Paper 26.2 configuration.

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Target Minecraft Versions: all currently supported versions
Requirements: none
Related Issues: #113

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