Aender singleplayer and multiplayer verification

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Goal

Verify the current Aender progression, portals, scaling, terrain regeneration, and stabilization behavior in realistic play.

Test matrix

• Fresh singleplayer world
• Existing or upgraded world
• Dedicated server with a separate client
• Two or more simultaneous players
• Reconnects, death, and repeated portal travel
• Travel in both directions and near world-border limits
• Volatile terrain with stabilized and unstabilized chunks
• Automatic counterpart portal creation

Record versions, reproduction steps, logs, screenshots, and any discovered regressions in linked issues.

Target: v0.3.0