Reduce arena preparation memory spikes

[GerwinVerkerk]

Describe your changes

Stream arena blocks lazily during preparation instead of first allocating a List<Location> containing every block in the arena.

For a 1000 x 100 x 1000 arena, the previous implementation attempted to create 100 million Location objects before the per-tick preparation task began. That creates a large, immediate heap spike in GameRegion#getBlocks() and can freeze or exhaust the server before rollback logging has made progress.

This change:

• adds a lazy block iterator to GameRegion;
• makes PrepareArenaTask consume blocks directly at the configured per-tick rate;
• applies predicates while iterating, so bonus-chest-only scans are lazy too;
• preserves the existing traversal order, rollback snapshots, bonus block handling, and scheduler cadence.

This removes the eager location-list allocation reported in #113. Rollback itself can still retain many BlockState snapshots for very large arenas; changing that storage model is a separate architectural concern and is intentionally outside this PR.

Verification

• ./gradlew build
• 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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