Replace Havok-dependent navmesh debug display

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

The issue names no files, tests, or entry points. Start by locating the current Havok-based navmesh debug display and the simulator/viewer pathfinding-data protocol, then trace how older simulator versions are handled. Done means an equivalent display works without Havok, performance is maintained, compatibility is preserved, and the format and protocol are documented.