Fix #817: Copernicus tile key FP precision for N03/N15/N63 bands

[wilhel1812]

Summary

• Fixes terrain data missing for entire latitude bands N03, N15, and N63 (including Trondheim area)
• Root cause: Copernicus DEM GeoTIFF files have ModelTiepoint Y coordinates with floating-point precision errors (e.g., 63.99999999999999 instead of 64.0), causing Math.floor(minLat) to produce wrong tile keys
• Solution: Replace Math.floor with Math.round for latStart and lonStart in both the worker and main thread parsers

Changes

• src/workers/copernicusTileParser.worker.ts: Math.floor → Math.round
• src/lib/copernicusTerrainClient.ts: Math.floor → Math.round

Verification

• All terrain-related tests pass (copernicusTerrainClient, srtm, copernicus proxy)
• Verified GeoTIFF tiepoint values for affected bands via direct S3 inspection