Viewport splitting, USD import/export, rendering and input refactors, training/UI polish, platform fixes,nd C aI/version metadata cleanup.

What's Changed

• native file dialog by @MrNeRF in #1013
• Nanogs by @MrNeRF in #1014
• Reuse ppisp by @MrNeRF in #1015
• Improved Input handling by @MrNeRF in #1026
• Rendering refactor by @MrNeRF in #1000
• windows: fix tests linking by @miriameng in #1030
• LFS densifcation by @MrNeRF in #1031
• Usd format import export by @MrNeRF in #1032
• windows: fix test linking by @miriameng in #1034
• Depth mode selection by @MrNeRF in #1037
• Viewport splitting by @MrNeRF in #1043
• Fix mcmc defaults by @MotivaCG in #1046
• Add --output-name parameter to override default PLY filename by @MotivaCG in #1048
• windows: fix tests linking by @miriameng in #1052
• Improve for aarch64 environment by @todashuta in #1053
• MCP: Add full UI window capture, harden viewport capture by @SharkWipf in #1050
• Fixed the build‑failure issue on environments that do not support AVX2 by @todashuta in #1062
• Fix modal click-through on startup overlay by @jacobvanbeets in #1057
• Fix Windows default app association to use system Settings UI by @jacobvanbeets in #1061

Full Changelog: v0.5.0...v0.5.1