[codex] Optimize capped elevation LOD lookup

[jasonbeverage]

Summary

This optimizes ElevationPool::getLOD for callers that immediately cap the returned LOD by requested sampling resolution. The helper now accepts the maximum useful LOD and stops searching a layer's data-extent R-tree as soon as it finds a level that meets or exceeds that cap.

This preserves the returned sampled LOD after the existing std::min(...) cap, while avoiding unnecessary R-tree callbacks in data-extents-heavy elevation layers.

Benchmarks

Added BM_ElevationPoolSampleMapCoordsFixedResolution, which samples 4096 map-coordinate points against a constant elevation layer with 256 overlapping data extents and validates that every point is sampled successfully.

Command, from tests after osgearth_shell.bat:

osgearth_benchmarks --benchmark_filter=BM_ElevationPoolSampleMapCoordsFixedResolution --benchmark_min_time=1s --benchmark_repetitions=3 --benchmark_report_aggregates_only=true

Before:

BM_ElevationPoolSampleMapCoordsFixedResolution/4096_mean  2897964 ns  2823795 ns

After:

BM_ElevationPoolSampleMapCoordsFixedResolution/4096_mean   218933 ns   219739 ns

That is roughly 13x faster on this targeted path.

Validation

• configure.bat
• build.bat
• Focused benchmark above
• ctest --test-dir build -C RelWithDebInfo --output-on-failure reports no registered tests
• Documented osgearth_tests command could not run because no osgearth_tests executable was produced or installed in this build tree