Optimize world generation using parallel processing

[Vianpyro]

No description provided.

[Copilot]

Pull request overview

This PR parallelizes several world-generation hot loops using Rayon to reduce end-to-end generation time for large maps (default 10,000×10,000).

Changes:

• Parallelize elevation grid generation by computing rows concurrently.
• Parallelize terrain classification and biome classification by processing rows concurrently.
• Parallelize per-city resource aggregation across city slots.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/terrain.rs	Uses par_iter_mut() to classify terrain rows in parallel.
src/elevation.rs	Uses par_iter_mut() to compute elevation rows in parallel.
src/biome/generation.rs	Uses par_iter_mut() to compute biome rows in parallel.
src/biome/city_resources.rs	Uses par_iter() to compute per-city resource summaries in parallel.

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