⚡ Optimize terrain page generation loops

[google-labs-jules]

💡 What:

• Optimized getTerrainPage in WorldService to reduce overhead in parsing and looping.
• Implemented a fast path for parsing single-value strings (flat nodes) without split/map allocations.
• Added a fast execution path for flat nodes (single texture/height value) to avoid per-pixel array lookups.
• Hoisted loop invariant calculations (zOffset).

🎯 Why:

• The terrain generation loop iterates 128x128 times per page. The original implementation performed string splitting and bounds checking inside the loop, which is inefficient for the common case of flat or uniform terrain chunks.

📊 Measured Improvement:

• Benchmark showed a ~1.20x speedup (approx 20% reduction in execution time) for processing 1000 nodes with a mix of flat and detailed chunks.
• Validated with new regression tests backend/src/world/world.service.terrain.spec.ts.

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PR created automatically by Jules for task 1528773312047276050 started by @7185

[google-labs-jules]

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