Replace Havok-dependent navmesh debug display

[bennettgoble]

Description

Research and implement a replacement for the current Havok-based navmesh debug display system to eliminate Havok as a dependency from the Second Life viewer.

Details

The Second Life viewer currently requires Havok libraries solely for rendering the navmesh debug display - a feature used primarily by content creators and region developers to visualize pathfinding meshes. This dependency adds a hefty maintenance and open source overhead for a rarely-used feature (The physics extensions package are notoriously annoying to update, and we have to keep havok libs in a private gdrive for TPVs that have signed the havok sublicense.) By converting the navmesh data from Havok's proprietary format to our standard llmesh format (or another general-purpose format) at the simulator level, we can remove this dependency entirely while maintaining the functionality.

Acceptance Criteria

• Havok dependency completely removed from the Second Life viewer codebase
• Navmesh debug display remains functional with equivalent or better visualization quality
• Performance of navmesh visualization is equal to or better than current implementation
• Backward compatibility maintained for regions running older simulator versions
• Documentation updated for the new navmesh format and protocol

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