Fix data corruption by updating region label storage from u16 to u32

[Vianpyro]

No description provided.

[Copilot]

Pull request overview

Updates the chunk payload encoding in the world save format to prevent region label truncation/corruption by storing per-tile region labels as u32 rather than u16.

Changes:

• Write per-tile region_label as 4 bytes (little-endian u32) in chunk payloads.
• Read per-tile region_label as 4 bytes (little-endian u32) when loading chunks.
• Update top-of-file format documentation to reflect 8 bytes/tile chunk storage.

Comments suppressed due to low confidence (1)

src/save.rs:219

• raw is now populated with 8 bytes per tile (1 terrain + 2 elevation + 4 region label + 1 biome), but the preallocation is still sized for the old 6-bytes/tile layout. Update the Vec::with_capacity(...) multiplier to avoid avoidable reallocations while encoding chunks.

                    raw.extend_from_slice(&(data.region_labels[idx] as u32).to_le_bytes());

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